Harley Benton GA5: cheap Valve Junior clone + EQ

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Post by modman »

I really enjoyed reading about Brigg's Valve Junior -> Supro conversion thread, and I saw that http://www.thomann.de is selling a clone of the VJ that has an EQ and is even cheaper at 98EUR (88EUR for head), I jumped on the first one for sale by a local blues harp player. He'd bought a bunch of small 5W combo's and sold off the ones he didn't like, also sold an Fender Champion for 190 EUR but preferred the Harley Benton soundwise.

And excuse me, but with the cream coloured tolex, leatherette handle, it looks really nice :thumbsup . Shame about that electric heater style on/off pilot button :?

Image

After you've taken the back panel off, similarities with the VJ begin to show:

Image

Stock tubes are Sovtek for the powertube EL84 and some unbranded 12AX7 Chinese preamp tube:

Image

Here's a look at the pcb board showing the two tubes and the connection to the tone control that not there in the Epi VJ. Also note that capacitors are gooped to the board, and the tone control pcb and output jacks are also gooped to the chassis.

Image

In fact the circuit perfectly matches that of the Epi VJ, except that R15 1k5 is to be replaced by the tone stack.

Epiphone Valve Junior stock schematic

Here's a see through close up of the Benton pcb.

More interesting perhaps is the tone circuits, which I think hasn't been published before, so I did some effort tracing that in the hope of finding what made up this weird "FAT BASS EQ was. The input an output and together with a ground connection plugged integrated into some slide plug. One correction: the EQ pot is 1M log :shock: , not linear, which might explain why is hardly efficient on the treble side.

GA5 Tone Control See Through PCB

Which I tried to put down as this:

Image

Cannot see the point of the 10 ohms - brown black black gold - and just checked with the DDM - 10ohms it is!

And cannot be terribly wrong because it looks alot like this standard Marshall tone stack if you think the treble pot is 120k fixed resistor,

Image

Will be looking into the standard Epi VJ mods soon, already changed the input resistor to 33k but I only have 1/4W 1Ms here now, wonder will that hold for pull down resistor? Will be playing with the Tone Stack emulator tomorrow too.

Oh how it sounds? Well, most importantly it's very silent (reports on VJ were terrible initially, but I think they added two caps to the initial design). Speaker is not the best, but once it heats up it really get a nice at home tube tone. Perfect for screaming blues leads with a booster or OD pedal already, but I feel there's room for improvement. Still haven't tried in on my 2 x 12". Important difference with the Epi: it has only 4 ohm out.

Now I really got to get to bed.
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Post by JiM »

Hi Modman,

I have exactly the same amp, and already applied some mods from the abundant litterature about the Valve Jr.
It sounds great !

The GA-5 IS indeed a Valve Jr. (this reference on the PCB is imperfectly scratched on mine), certainly built in the same chinese factory with slightly different specs (outside look, tonestack).
The HP really looks like the Weber 8" used in the VJ, if not original it's a good copy. All the early flaws in the VJ circuit (hum from heaters, bad grounding, insufficent power supply filtering, etc.) were corrected in subsequent revisions, i guess the GA5 is equivalent to VJ rev.3 with DC heaters.
Modding is quite safe on this amp as there is a bleeder resistor on the power supply, discharging the deadly 300V+ in filter caps in seconds. But don't rely on it, and always check with a multimeter, with one hand in back pocket ! I also use a resistor taped on a wooden stick for safe cap discharge.

I know you speak french, you can find a lot of information (but with a much lower signal-to-noise ratio than here) on the audiofanzine forums :
http://combo-guitare.harley-benton.audi ... ,ga_5.html
http://forums.audiofanzine.com/index,id ... _live.html
http://forums.audiofanzine.com/index,id ... n_ga5.html
(yes, there is quite a lot to read !)

The tonestack schematic was already published here : http://www.ringo.com/photos/photo.html? ... =245134429, but with mistakes : treble cap is noted 47pF instead of 470pF and the strange 10 ohm resistor is noted 10k. I also had to double-check its value, and i confirm it's 10 ohm. I already played with Tone Stack Calculator, and 10k would definetely be more natural and 3dB louder.

The mods i have done :
- changed the preamp tube : this chinese 12AX7B was full of microphonics, i used the original ECC83 from my Valvestate, then swapped again for a Sovtex 12AX7WXT+ (the Marshall got it's old tube back :wink: )
- replaced input resistor to ground (R1) from 68k to 1M. Gives a great boost and allows the preamp to crunch. Highly recommended ! I also had only 1/4W 1M resistors, and i think it would be fine, as there is almost no current here.
- suppressed the tone stack (just put a 1k5 in the connector between red and yellow wires). You get a stock Valve Jr., with a lot more volume, brighter tone, better dynamics ... one drawback : 5W is LOUD ! :shock:
- re-used the tone pot as a master volume, wired as variable resistor parallel to R5. (Will redo this, replacing R5 and wiring the pot as voltage divider, for more progressive control). Now the volume is acceptable even at full preamp gain, but you loose the power tube overdrive at low volume.
- added a 1k 1W resistor on the screen of the power tube (pin 9, i soldered it "SMD-style" under the PCB, you need to cut a track), i was told it's required to avoid ealy tube wear. No change in sound.
- breadboarded a bridged-T power attenuator (see my previous post on Briggs' topic), it's a must-have for late nite playing in appartment, and retains the power tube overdrive. Also a useful option, i still hesitate to build it as a separate output jack or with a (rotary) switch.
- tried some negative feedback, but still need some serious A/B test.
- tried replacing the EL84 with NOS russian 6N1P (with a dirty homemade don't-put-your-fingers-here adapter) for 1W output. More hum, not much less volume, still need to test that.

Mods i will do :
- triode switch for the EL84.
- standby switch (maybe the same as triode with a on-on-on DPDT)
- reduce B+ for longer tube life and use with 6N1P. Maybe a power damping here instead of output attenuator ?
- try Marshall or Vox mods, or maybe the Supro ones posted by Briggs. Any suggestions ?
- Conjunctive filter like the Minicat, i'm curious of what this does to the sound.
- add a one-knob tone control like the "contour" pot of the early Valvestate. Or tune the original tonestack, with bypass switch.
- line out with Condor cab sim (from runoffgroove.com)

Other useful mods :
- 8 ohm output : the red wire comes from the output transformer, you just have to drill a hole and solder a jack. I don't need it as i have only 4 ohm speakers. Indeed the GA5 sounds great on a Celestion G12T-100 :twisted:
- hi/lo inputs just like the original Gibson GA5
- bright switch
- messing with R6 and R7 for more gain
- add a tube tremolo

Thanks for the "see-thru", it will help a lot !

Hmm, i need to sleep too ...
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Post by marshmellow »

Hi modman,

there is already a schematic, I think I got it from the german Tube-Town-Forum. Look in the attachment.

You can use the 1/4W 1M resistor, no problem at all. Actually grid current is near zero.
As JiM already mentioned there should be another 8 ohm tap on the OT. Just install a switch or another jack there.

Also worth a thought might be getting a better/bigger OT as Briggs did. Should be a reasonable improvement. The stock one is really a bit overstrained, especially with too much bass (as is the speaker..).


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Post by DWBH »

JiM wrote:Hi Modman,
- replaced input resistor to ground (R1) from 68k to 1M. Gives a great boost and allows the preamp to crunch. Highly recommended ! I also had only 1/4W 1M resistors, and i think it would be fine, as there is almost no current here.
- suppressed the tone stack (just put a 1k5 in the connector between red and yellow wires). You get a stock Valve Jr., with a lot more volume, brighter tone, better dynamics ... one drawback : 5W is LOUD ! :shock:
- re-used the tone pot as a master volume, wired as variable resistor parallel to R5. Now the volume is acceptable even at full preamp gain, but you loose the power tube overdrive at low volume.
I have the same amp, GA5. My best buy ever.
I've done the same mods above.

Next mods: Replace tubes with better ones. Regulate B+, as I think that's what is causing that rattling distortion.

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Post by JiM »

Hi,
I've plotted the action of the tone control with Tone Stack Calculator, here it is.
The curves show 0%, 5%, 20%, 50% and 100% of the rotation of the pot, 20% being blue. I did not check if the input and output impedance are right for the GA5, just kept default values.

There's quite a big notch at 800 Hz due to the Mid "pot" being completely down, and the Bass control has most impact in the first part of its travel (despite being log already).

One could adjust the resistors emulating Hi and Mid pots to taste, based on a standard Marshall tonestack. Or just add these pots.
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GA5_tone_response.png
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Post by analogguru »

A typical schematic which shows when non-technicians go to work.... motto: "...and they dont know what they are doing...."

The 68k/68k voltage divider is from a time when there were two inputs - LO/HI - on the guitar amps (look at old Marshall schematics). If the guitar was plugged to the High-level input the signal was attenuated (to avoid necessary readjustments of the controls).

Forget this shit: replace the 68k to ground with a 1M resistor. this will give you nearly the double signal.


The same with the 1M resistor after the decoupling capacitor of the first stage. This resistor was implemented with a parallel capacitor to make a treble boost. without the cap it lowers only your signal by the factor of three.

Forget this shit too: Jumper this 1M resistor and your signal will increase by the factor of three.

With this mods you can drive the second stage - with a six time higher signal - into distortion. If your Amp gets too loud then add a Master-volume-(trim)-pot after the EQ.

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Post by JHS »

The stock Valve-Junior and the HB suffer from cheap output transformers and electrolytics.
If you want to make this thing sound good replace those cheap caps and install a better output transformer (f. ex. a Hammond 125CSE). To prevent sub-harmoics change the value of the first filter cap to 50-100uF.

As Analogguru recommended, change the 68k/68k combination for a single 1M and drop the 1M pad-R on the vol-pot.
To prevent an oversaturated sound drop the 22uF bypass electrolytic on V1b.
Install Orange drops or Mallory 150 coupling caps.

A 50p SMica parallel to the 1M will give a slight treble boost, acc. to the used preamp tube this can be useful.

Furthermore I would swap the tubes for some Tesla or Siemens.

JHS

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Post by modman »

JiM wrote: The GA-5 IS indeed a Valve Jr. (this reference on the PCB is imperfectly scratched on mine), certainly built in the same chinese factory with slightly different specs (outside look, tonestack).
The HP really looks like the Weber 8" used in the VJ, if not original it's a good copy. All the early flaws in the VJ circuit (hum from heaters, bad grounding, insufficent power supply filtering, etc.) were corrected in subsequent revisions, i guess the GA5 is equivalent to VJ rev.3 with DC heaters.
That's the first thing I remarked: the power circuit must be improved in comparison to the ealy valve junior schematics circulating around, it's dead silent.

Modding is quite safe on this amp as there is a bleeder resistor on the power supply, discharging the deadly 300V+ in filter caps in seconds. But don't rely on it, and always check with a multimeter, with one hand in back pocket ! I also use a resistor taped on a wooden stick for safe cap discharge.

Thanks for the links, aucun problème!

I have done the following changes so far:

R1 = 1M pull down instead of the 68k
R2 = 33k that I could have lowered even down to 10k, but as analogguru points out, that heaps of gain in reserve, so figured I'd go with the Princeton Tweed value
R6 = jumper
R8 = 1k5
R9 = 2k2
C3 = 4.7uF
C4 = 4.7uF

Stock biasing I caculated at 11.3W to 11.6W - took several measurements, they fluctuated somewhat through the evening and the following morning.

The extra jack for the 8 ohm output is really essential because I might mostly be using this and on 8 ohm Fender like speakers, so will have to get the drill out. Was prepared for that because I'm really planning some real mojo on the tonestack - will just put the Valve Junior 1k5 there, remove the first decoupling capacitor and plug this in front of the gain:

Image

But should I use 400V caps if I implement it between the stages?

analogguru wrote:A typical schematic which shows when non-technicians go to work.... motto: "...and they dont know what they are doing...."

The 68k/68k voltage divider is from a time when there were two inputs - LO/HI - on the guitar amps (look at old Marshall schematics).
and 5D2 ... for example

Yes, it is absolutely amazing how they eventually cleaned up the power section in the Epiphone and nobody at Thomann who were intelligent enough to clone the corrected schemo but still didn't see the total mismatch input.

But really no escaping a master volume control, I think. Still haven't gotten round to scoring a decent EL84 either.

thanks for the comments!
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Post by floris »

I also found some info on the web that stated to put the R1 (changed to 1M) resistor directly from signal input to ground. R2 then functions as a "grid-stopper". R1 is the input resistance for the stage before the amp (guitar or fx). R1 and R2 will not form a voltage divider then. I did this change on my VJ and heard no audible difference (as far my memory enabled me to compare). It (still) sounds good. I did not see this suggestion in many of the mod info available on the web.
See also:
http://www.aikenamps.com/InputRes.htm
"...
The grid resistor accomplishes the following things:
# It helps prevent high frequency parasitic oscillation in the tube itself
# It helps prevent radio frequencies from getting into the input stage, where they can be rectified and lowpass filtered (AM detection) and become audible at the amplifier output
# It can limit grid current when the tube is driven into the positive grid region, which helps in preventing "blocking" distortion
..."

Any ideas or experience on this one?
If I would have known beforehand, I would have bought myself a Harley Benton GA5! They are cheap!

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Post by JHS »

If you have a good preamp tube you don't need a grid stopper, w/o it the sound is a bit more natural.
A lot of good sounding old tube amps, Hiwatt for ex.and a lot of tube FX, have no grid resister.

JHS

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Post by JiM »

Regarding grid stopper : i kept R2 stock, 68k between input and R1 (1M to ground). Should i expect something different if i move R1 between input and R2 ?

About Modman tone stack : i think you will need to use high-voltage caps here, as you intend to remove the coupling cap : the tonestack is basically 3 coupling caps ! The input will be at about 200V above ground, and the output will be referenced to ground. By the way, i've simulated it roughly, it looks nice. I'd lower the mid cap a bit though, for example 22nF, or is it a low-mid on purpose ?

About B+ : the anode of the EL84 is at 313V above ground, the cathode being biased at 10V. According to different sources, this is about right for such an amp, and reducing B+ is needed only for even higher voltages. But the EL84 datasheets recommand 250V, and list 300V as an absolute maximum rating ... At 303V i'm a bit above, should i do something ?

Yesterday i tried switching some negative feedback in and out, and i did not hear significant difference. I did connect the 8 ohm tap of the OT (the HP is on 4 ohm tap) to pin 8 of the preamp tube with a 47k resistor. Is that right ? What change in tone should i expect ?

I also tried shunting R6 for more gain, it works well, and looks like second triode gets involved in a thicker overdrive. I did a better wiring of the master volume, it's a lot more progressive now.

I still miss some sparkle when preamp is fully cranked (master lowered, and bridged-T attenuator on output), but i re-discover the volume controls on my Epi SG ! With treble-bleed caps and single-coil switches mods, i can have either shiny or creamy tones at will 8)

Next steps : triode mode, serious attenuator wiring, looking at Briggs' Supro, maybe a bright switch ...
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Post by modman »

thanks, I put this one in the list with tube site links.

Thanks for so many comments and links. Do want to point explicitly to the documents in the link JiM posted, which contains a hefty PDF detailing the whole Harley Benton circuit:
http://thepulse.chez-alice.fr/Descripti ... Junior.pdf

Beside the changes listed above, I also
-converted the EQ pot to a Master Volume using the 1M log pot
-switched the gain to a linear taper (B), don't know if that was smart, it's a bit steep
-put the 3 knob tone stack on a piece of perfboard and drilled out three holes in the back for my secret weapon tone stack.

Only didn't put in a swtch to bypass the tone circuit because I measured 126V in the tone circuit, don't know how do that safely and without popping perhaps the speaker to shreds...

At last some control over those wretched lower mids. Still I can hear the limitations of the speaker, now where my 2 x 12" ?

It's going to get an extra 8 ohm out jack too, but don't have any mono's, so will have to use two stereo's of a different to get them aligned.

Heaps of fun for 98 EUR and cannot wait to put it next to a friends boutique princeton with massive transformers and a KT66 once more -- even without getting custom wound ex communist transformers. :twisted:

to be continued, maybe I will order the hammond transformer. Getting decent tubes already cost me almost 1/2 of what I paid for the amp :hmmm:


EDIT: didn't see your post JiM, sorry:
JiM wrote:Regarding grid stopper : i kept R2 stock, 68k between input and R1 (1M to ground). Should i expect something different if i move R1 between input and R2 ?
French PDF says something about that, the sequence of input resistor and pull down. Don't know whether to believe it though, hop
About Modman tone stack : i think you will need to use high-voltage caps here, as you intend to remove the coupling cap : the tonestack is basically 3 coupling caps ! The input will be at about 200V above ground, and the output will be referenced to ground. By the way, i've simulated it roughly, it looks nice. I'd lower the mid cap a bit though, for example 22nF, or is it a low-mid on purpose ?
it's the tone stack of my favorite distortion box: ROG Umble, and I really just plucked it out of the scheme. Like I said, I really tames all the muddiness, Will experiment with the mid cap value though. Am using 250V wima's except for the 330pF which seems to be holding out fine - but for how long?
About B+ : the anode of the EL84 is at 313V above ground, the cathode being biased at 10V. According to different sources, this is about right for such an amp, and reducing B+ is needed only for even higher voltages. But the EL84 datasheets recommand 250V, and list 300V as an absolute maximum rating ... At 303V i'm a bit above, should i do something ?
I'm at 308V, but if I remember correctly, Briggs' was still higher. Check in that thread again.
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Post by JiM »

modman wrote:It's going to get an extra 8 ohm out jack too, but don't have any mono's, so will have to use two stereo's of a different to get them aligned.
Would a flexible output wiring be feasible ? One 4ohm output, one 8ohm output, and using appropriate switching sockets when both are plugged you get 4ohm each, either in series on the 8 ohm tap, or half seconday coil each.
modman wrote:EDIT: didn't see your post JiM, sorry:
No problem. I saw you were online ...
modman wrote:French PDF says something about that, the sequence of input resistor and pull down. Don't know whether to believe it though, hop
In fact this PDF is basically a french translation of this document from S2amps : http://www.s2amps.com/docs/vj_kit_inst.pdf and refers to the first version of the Valve Jr. (AC heaters). I've already found more precise informations on the sewatt forum !
Right now on my GA5, R2 is shunted (so no matter the position of R1), there is a 560pF cap across R6, and i've got all the lost treble back ! It's very sensitive on the guitar controls (esp. volume and tone pots ... plus my secret weapon : a passive schottky clipper). I've got a new guitar since i'm modding this amp :thumbsup
modman wrote:
I'd lower the mid cap a bit though, for example 22nF, or is it a low-mid on purpose ?
it's the tone stack of my favorite distortion box: ROG Umble, and I really just plucked it out of the scheme. Like I said, I really tames all the muddiness, Will experiment with the mid cap value though. Am using 250V wima's except for the 330pF which seems to be holding out fine - but for how long?
I heard that most of the muddiness lies around 400Hz, and this mid control is centered around 25OHz. :roll:
I really should prototype these ROG designs, especially Umble, MatchBox and Supreaux. But right now i have a OC75-based RangeMaster clone on the bench ! Maybe i'll post some pictures and sound in an appropriate thread ... if it's worth.
modman wrote:
But the EL84 datasheets recommand 250V, and list 300V as an absolute maximum rating ... At 303V i'm a bit above, should i do something ?
I'm at 308V, but if I remember correctly, Briggs' was still higher. Check in that thread again.
I just had a silly idea today : on this side of the ocean, the simplest way to lower the power supply would be useing another primary tap on the power transformer. Mine is wired on 230V (blue wire) which is correct, but if i use the 240V tap (brown wire) on 230V mains i should get a more acceptable B+. Heaters will still be within spec, and i expect 320V B+, then about 290V across the EL84. Or am i missing something about tube biasing ? I'm really worried about this, the cathode glows red and the glass temperature gets as high as 150°C.
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Post by marshmellow »

Yes, it's not within the specs, but shouldn't make the anode glow. The 18W clones run at 320-340V.
I also tend to use the 240V tap, works still fine for the heaters. I don't know the history of your amp, are you still using the stock transformer?
Anyway, if the anode is glowing (as soon as you see this switch the amp off, doing this too long you can buy a new tube) you need to raise the cathode resistor, since your bias current is way too high.

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Post by JiM »

Hmm, the cathode glows red, not the anode hopefully !
Of course it needs to be hot to release electrons, but it looks a bit too bright to me, and quite hot at the surface of the glass tube, esp. compared to the preamp tube. Maybe it's completely normal, and i'm just too cautious.
BTW, i've found an old 6BQ5 datasheet specifying 250°C as maximum bulb temperature ... still 100°C left !
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Post by Entrant_21 »

I got my Harley Benton GA5 combo (repackaged valve junior with added bass EQ control) the other day..and i love it, amazing amp for the money! (£69+delivery) im drawing up a materials list for the mods ill be performing on it soon, then it came to me...why not add an fx loop? it shouldnt be too hard as the amp isnt SS, also it's working with a low wattage output so there shouldnt be much hassle adapting it to handle a loop. I understand that fx loops are placed between the pre-amp and power amp stages in the amp; so looking at the circuitboard the loop would be placed somewhere around C2 and R15 (if im following the traces right)

Has anyone attempted this before? I tried googling but only got sites talking about some speaker add on...

There has to be some form of attenuation on the loop's "SEND" jack as the voltage or current I would imagine is quite high..and then some sort of gain recovery stage after the "RETURN" jack but this is my first time working with valves so im still learning about their operating conditions and characteristics.

Anybody care to share any thoughts? I really want this feature on my amp!:D

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Entrant_21
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Post by Entrant_21 »


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JiM
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Post by JiM »

and i love it, amazing amp for the money
Me too ! And it's even better after some mods :D
I recommand removing the tone stack (and re-using the pot as a master volume), changing R1 to 1M, and adding a power attenuator on the output (if you want to crunch at home and/or at night).
why not add an fx loop?
I asked this question too, and i went to the conclusion that it is not really useful on such an amp. Why ? Because the great tone you get from such a low-power class-A tube amp is based on slight overload of each gain stage, including the power tube, and maybe the output transformer and HP too ...

The purpose of the FX loop is to place time-based effects (the ones that get muddy if distortion occur after them) between the tone-shaping/distorting preamp and a big-headroom power amp. On this one, this would mean to put the FX loop after the power tube (and fry your pedals), or after the HP (i.e. mike the amp). :?

On the schematic you've found, the loop is just after the first gain stage (yes, in the middle of the preamp), so i guess there would not be a great difference compared to effects in front : most overdrive happen in the second stage, overloaded by the first one.

I'd like to be proven wrong, though, as it's a simple mod. Please tell us how it sounds !
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Post by modman »

69 EUR for the head version on ebay Germany
http://cgi.ebay.de/Harley-Benton-GA5H-N ... otohosting
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