Peavey Backstage Plus 35W Guitar Amp  [schematic]

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Post by DrNomis »

Hi Everyone,


I've just recently been given a Peavey Backstage Plus 35W Solid State guitar amp for nothing, and I'm currently working on it to get it back to a useable state, so, what's wrong with it?

Well, the original speaker is a no-worker but the rest of the amp seems to work fine, I verified this by plugging my pair of headphones into the headphone socket, I get normal sound through my headphones but no sound through the speaker.... :thumbsup


I've managed to find what I think should be a good replacement on the Musician's Friend website, the replacement speaker I have selected and ordered online is a 10 inch Jensen C10Q speaker.


Also managed to find some good/readable schematics for it online too...... :thumbsup


The other thing wrong with the amp is R63, a 22 Ohm /1W resistor, is burnt out so I'll be replacing R63 with a new one..... :thumbsup


The only other thing the amp needs is all the pots sprayed with contact cleaner, which I've already done..... :thumbsup


Finally, one mod I'm going to make to the amp is to replace the original power indicator led with a blue one...... :thumbsup

Another mod I'm going to do is replace all the 4558 op amp ICs with TLO72CP....... :thumbsup


And also make a footswitch for the amp as well..... :thumbsup


Anyway, just thought I would post the schematics for any one interested in downloading a copy:
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Peavey Backstage Plus Schematics.pdf
Peavey Backstage Plus Schematics
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Post by DrNomis »

Update:


Replaced the burnt-out R63 resistor, and done all the mods I wanted to do, everything went fine so now all I'm waiting on is the new Jensen C10Q replacement speaker..... :thumbsup


Also made a footswitch for my Peavey Backstage Plus amp as well since both the Saturation (Distortion) and the Reverb are footswitchable, this amp is shaping up to be quite a useful little amp, been surfing the Internet for info about this amp and have read lots of good things about it, apparently it's a loud amp for it's size and power, also read that it works well with stompboxes...... :thumbsup
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Post by DrNomis »

Update:


Three of the pots have failed, basically the carbon paint on the resistive track has flaked off so the pots need replacing, the pot for the Saturation control is a dual-gang type but one of the resistive tracks is 1k so it's not as simple as just replacing it with a dual-gang 10k pot, the other one is 10k, the Post control pot, 10k, also needs replacing as well as the reverb pot,10, have tried seeing if some pots from Jaycar would work, they won't.... :hmmm:


Just wondering where I can buy a complete set of new replacement pots for my Peavey Backstage plus...... :hmmm:
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Post by bajaman »

those pots are crappy - try your local Peavey agent for spares :wink:
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Post by DrNomis »

bajaman wrote:those pots are crappy - try your local Peavey agent for spares :wink:

Will do, been doing some googling for new pots too, but looks like I'll probably have to see if I can order some from Peavey USA, was hoping I could order them from a Peavey agent in Australia, I'm pretty keen on getting my Peavey Backstage Plus going again since it was sounding good before the pots failed, the Jensen C10Q speaker I bought for it was a great choice..... :thumbsup
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Post by clintrubber »

Hi,

Curious if you managed to find that non-standard dual pot for the saturation-position?
Have a 'non-Plus' Backstage upcoming which uses the same pot, and it seems to be scratchy...

Thanks

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Post by microbailey »

I'm currently modifying an early 00's Peavey Bandit (see the thread "My mission to modify a Peavey Bandit into a boutique amp" I've got going on here) and I'm replacing a couple of the crackly pots.
I know for that amp the Marshall spare pots are the same as Peavey used originally.
I don't know where you're based but in the UK I buy my replacement pots (including Peavey/Marshall) from Hotrox (hotroxuk.com)
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Post by clintrubber »

microbailey wrote: from Hotrox (hotroxuk.com)
The amp has arrived, pots are indeed a bit scratchy, but all functional, so hope just some spraying will do for now.

Thanks for the Hotrox-info, might come in handy... Might be good that I have a look already for a spare saturation-pot since that'll be the most difficult part to obtain.

Good luck with the Bandit!

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Post by clintrubber »

FWIW: having a Backstage 30 and a later Backstage (coloured knobs, reverb, satur.-circ.) side by side,
it's interesting/boring/(ir)relevant to see the changes they've made:

-to my surprise the cabinet height of the latter is slightly less.
-the alu side-trims on the baffle are smaller for the latter, and seem to be plastic now :shock:
-'rougher' (more 3D) tolex
-the nice 3D front panel of the '30' changed into a simple silkscreened version for the latter
-both logo's seem plastic, but I understood a metal logo also existed - perhaps on the bigger amps only ?
-I guess I don't have to mention the change from the ubercool Sputnik-style alu-knobs to the conveniently coloured but fully unvibey plastic knobs :hmmm:

Al in all, the latter seems to be a bit more streamlined... in other words, where production costs could be saved
they've done so. That's how things go.

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Post by DrNomis »

clintrubber wrote: 30 Mar 2018, 14:43 Hi,

Curious if you managed to find that non-standard dual pot for the saturation-position?
Have a 'non-Plus' Backstage upcoming which uses the same pot, and it seems to be scratchy...

Thanks

Actually no, I really tried my hardest to find a replacement 10k/1k dual-gang pot but unfortunately I couldn't find one anywhere, so I ended up replacing the stock one with a single-gang 10k pot, as far as I can remember I only needed to make a very minor mod to the circuit to make the single-gang 10k pot work virtually the same as the original 10k/1k dual-gang pot, I need to refresh my memory by examining exactly what I did to make a 10k single-gang pot work in the circuit.

It seems to me like Peavey were very notorious for using non-standard dual-gang pots in these old 80's (I'm assuming that's when my Backstage Plus amp was made) guitar amps, whether it was on purpose or not, I don't really know for sure, but it sure makes servicing these things a bit harder than necessary, especially when Peavey no longer manufacture them.


Update:

I just figured-out how I did the very minor mod to my Backstage Plus amp so that a standard 10k 24mm single-gang pot will work perfectly well as a replacement pot for the original 10k/1k dual-gang pot, so here's a short write-up of how to do the mod:

Components/Parts needed:

X1 10k Lin 24mm Pot (I used a 10kB 24mm pot I bought from my local Jaycar Electronics store, seems to work perfectly well).

X1 1k/1% Metal Film 1/2W resistor (This replaces the missing 1k half of the original pot).

X7 Replacement knobs with grub-screws to fit all the pots.

X1 Small roll of tinned copper wire.

Here's what you do.....

Step 1....Remove the amp's circuit board from the chassis, after first making sure that there's no power going to the amp.

Step 2...Desolder the original 10k/1k dual-gang pot.

Step 3....On the copper-foil side of the circuit-board locate the solder-pads where the 1k half of the original 10k/1k dual-gang pot was soldered to the circuit board, and solder the 1k/1% metal-film resistor in place so that one end of the resistor is soldered to the pad that connects to C7 (22uF/25V), and the other end is soldered to the pad that connects to the two clipper diodes (CR5 and CR6), make sure you link the centre pad to this pad as well by using some of the 1k/1% resistor's lead as a wire-link.

Step 4...Cut the 10kB single-gang pot's shaft to length so that you can fit the new knob to it.

Step 5...Cut three 50mm pieces of tinned copper wire and solder each of these to their respective terminals on the new replacement 10kB pot, making sure you don't overheat the terminals or the pot.

Step 6...Solder the new 10kB pot in place on the circuit board, making sure that you allow enough tinned-copper wire length for the new pot to sit flush against the chassis where it pokes through the mounting hole.

Step 7....And finally re-install the circuit board back in the chassis before installing the new knobs on all the pots, once the amp is put back-together again give the amp a test-out, you should notice that the Saturation control should work virtually identically as it used to with the original pot.


Here's a tip, Peavey seem to really like using the JRC4558D dual Op Amp IC in these amps, the JRC4558D tends to be a bit noisy and hissy, fortunately there's a good low-noise, pin-for-pin compatible replacement for these, the TLO72CP, I replaced all the JRC4558D ICs in my Peavey Backstage Plus amp with TLO72CP ICs, but I soldered-in IC sockets to make them easily replaceable if I ever need to, works a treat to make the amp nice and quiet, so quiet that when I first turned it on I thought I had damaged something on the circuit-board, but when I plugged-in my guitar, turned up the volume, plucked a string, i was pleasantly surprised.



Hope that helps.......

I'm attaching a PDF of the Peavey Backstage Plus just in case anyone else needs to do service work on one of these, it includes the circuit board layout too, also attaching a pic of my Peavey Backstage Plus.
Attachments
Peavey Backstage Plus_resized.jpg
Peavey-Backstage-Plus-Schematic.pdf
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Post by DrNomis »

Looks like my Peavey Backstage Plus amp is playing-up again, I was just trying it out to see if it still works, and here's what it is doing now:

* Random loss of volume to where the amp is completely silent.

* Amp goes quiet but still makes sound that's very distorted.

* When I turn the master volume up, it suddenly gets very loud.

* Sometimes the amp stays silent even though the power light is lit, turning the master volume knob up does nothing.

* The fault seems to be sensitive to vibrations.

So, I'm thinking that it could be a dirty pot or maybe a bad solder joint somewhere that's heat, or vibration sensitive.

Going to put it on the workbench and see what I can do to fix it today.
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Post by DrNomis »

Think I know what's going on, looks like I need to replace all the 1/4-inch sockets with some new 1/4-inch Cliff Jacks.
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Post by DrNomis »

A parcel containing a small bag full of brand-new Cliff Jacks turned up in the post yesterday, so, today's agenda is going to be devoted to firstly making a cup of some good coffee (very important...lol), and secondly, getting some of them installed in my Peavey Backstage Plus amp, and hopefully the amp will be fixed, and stay fixed.
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Post by DrNomis »

Update:

Replacing all the 1/4 inch sockets with some new Cliff Jacks seems to have fixed the amp completely, the volume doesn't drop out like it was doing before, so, my guess is that the socket for the FX loop or the headphone out may have some bad contacts in them.

I'm going to be popping into my local Jaycar Electronics store for a few things tomorrow, including a bag of some 5mm Leds, the Backstage Plus uses 1N4148 small signal silicon diodes as clippers to create the overdrive sound, I'm planning on swapping them all out for some 5mm Red Leds in order to soften the clipping a bit, a 1N4148 diode turns on at about .5-.7V, a 5mm Red Led typically turns on at about 1.2V, what's more, a Led has a more gradual turn-on knee compared to a 1N4148 small signal silicon diode, a friend of mine owns a Marshall Guv'nor overdrive pedal which he says sounds amazing, it uses Leds as back to back diode clippers.

Seeing as my new FNIRSI 1014D 100MHz Dual Trace Digital Oscilloscope has a screenshot function as well as a built-in Function Generator, we can do a quick breadboard experiment to compare the clipping characteristics of 1N4148 small signal silicon diodes with that of 5mm Red Leds.....stay tuned!!!
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Post by mauman »

DrNomis wrote: 20 Jan 2025, 10:18 Update:

Replacing all the 1/4 inch sockets with some new Cliff Jacks seems to have fixed the amp completely, the volume doesn't drop out like it was doing before, so, my guess is that the socket for the FX loop or the headphone out may have some bad contacts in them.

I'm going to be popping into my local Jaycar Electronics store for a few things tomorrow, including a bag of some 5mm Leds, the Backstage Plus uses 1N4148 small signal silicon diodes as clippers to create the overdrive sound, I'm planning on swapping them all out for some 5mm Red Leds in order to soften the clipping a bit, a 1N4148 diode turns on at about .5-.7V, a 5mm Red Led typically turns on at about 1.2V, what's more, a Led has a more gradual turn-on knee compared to a 1N4148 small signal silicon diode, a friend of mine owns a Marshall Guv'nor overdrive pedal which he says sounds amazing, it uses Leds as back to back diode clippers.

Seeing as my new FNIRSI 1014D 100MHz Dual Trace Digital Oscilloscope has a screenshot function as well as a built-in Function Generator, we can do a quick breadboard experiment to compare the clipping characteristics of 1N4148 small signal silicon diodes with that of 5mm Red Leds.....stay tuned!!!
Since two silicon diodes in series will give you about the same Vf as one red LED, I would have thought they'd be equivalent as clippers. But when I A/B test these two options, I usually go with the silicon. It seems I can always hear a little fizz from the LEDs when they approach their turn-off threshold on the trailing ends of notes. Someone theorized once that it was on/off/on/off oscillation right at that turnoff threshold for some reason, IDK but I always found it annoying. I hope you have a better outcome with the LEDs! It would be interesting to see if your scope catches anything like that.

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Post by DrNomis »

mauman wrote: 20 Jan 2025, 16:04
DrNomis wrote: 20 Jan 2025, 10:18 Update:

Replacing all the 1/4 inch sockets with some new Cliff Jacks seems to have fixed the amp completely, the volume doesn't drop out like it was doing before, so, my guess is that the socket for the FX loop or the headphone out may have some bad contacts in them.

I'm going to be popping into my local Jaycar Electronics store for a few things tomorrow, including a bag of some 5mm Leds, the Backstage Plus uses 1N4148 small signal silicon diodes as clippers to create the overdrive sound, I'm planning on swapping them all out for some 5mm Red Leds in order to soften the clipping a bit, a 1N4148 diode turns on at about .5-.7V, a 5mm Red Led typically turns on at about 1.2V, what's more, a Led has a more gradual turn-on knee compared to a 1N4148 small signal silicon diode, a friend of mine owns a Marshall Guv'nor overdrive pedal which he says sounds amazing, it uses Leds as back to back diode clippers.

Seeing as my new FNIRSI 1014D 100MHz Dual Trace Digital Oscilloscope has a screenshot function as well as a built-in Function Generator, we can do a quick breadboard experiment to compare the clipping characteristics of 1N4148 small signal silicon diodes with that of 5mm Red Leds.....stay tuned!!!
Since two silicon diodes in series will give you about the same Vf as one red LED, I would have thought they'd be equivalent as clippers. But when I A/B test these two options, I usually go with the silicon. It seems I can always hear a little fizz from the LEDs when they approach their turn-off threshold on the trailing ends of notes. Someone theorized once that it was on/off/on/off oscillation right at that turnoff threshold for some reason, IDK but I always found it annoying. I hope you have a better outcome with the LEDs! It would be interesting to see if your scope catches anything like that.

Yep, two forward voltage-drops from two series-connected 1N4148 small signal silicon diodes will add up to about 1.2V forward voltage-drop, but the knee where the diodes turn-on won't really change much, besides occurring at 1.2V rather than about .6V forward.

One thing I did notice with Leds is that the knee where they turn-on is more gradual compared to the knee of a 1N4148 diode, my old analog scope has a built-in component tester which can clearly show the knee in a diode's characteristic curves.

I think it would be really interesting to devote a whole forum thread on the topic of diode clipping characteristics, with screenshots and maybe FFT (Fast Fourier Transform) spectrum plots of the generated harmonics, I think my new Digital scope can do that too, but I need to get my head round the operating procedure to do it.
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Post by DrNomis »

Update:

Just worked out a very simple test-circuit based on a single TLO71CP Op Amp operating with a gain of X10 for testing/comparing the clipping characteristics of back-to-back 1N4148 small signal silicon diodes vs back-to-back 5mm Red Leds, I'll post some screenshots of the clipping characteristics tomorrow.
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Post by DrNomis »

Update:

As promised, I managed to get a couple of screenshots of the clipping characteristics of two back-to-back 1N4148 small signal silicon diodes versus the clipping characteristics of two back-to-back 5mm standard Red Leds, to be honest I was actually surprised at the results I got, there actually isn't that much difference between the two types of diodes used in this simple experiment, the main difference is the overall output signal level, here are the two screenshots, note the difference in output Vpp (Volts Peak to Peak), the 5mm Red Leds produce signal clipping at a higher Vpp signal output level, I will post a schematic of the test circuit I used to produce the results I got later on with component values and supply voltages.

Note that the spikes you see on the waveforms is due to the 100MHz bandwidth of my Digital Oscilloscope.
Attachments
This is a screenshot of the 1N4148 Silicon Diode Clipping Characteristics.
This is a screenshot of the 1N4148 Silicon Diode Clipping Characteristics.
This is a screenshot of the 5mm Red Led Clipping Characteristics.
This is a screenshot of the 5mm Red Led Clipping Characteristics.
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Post by DrNomis »

Update:

Just finished working on my Peavey Backstage Plus amp, after giving it a test-play I can say that changing all the clipper diodes from CR3 to CR8 from silicon diodes to 5mm Red Leds has definitely changed the whole character of the amp's distortion effect, it doesn't sound "boxy" like it did before, the amp sounds more open and less "strangled".

The clipping diodes to change are all the ones from CR3 to CR8, making sure that you have the Leds oriented the same way as the original silicon clipping diodes.
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