2 X 6V6GT amp build suggertions needed.

deltafred · Post by **deltafred** » 18 Apr 2012, 14:27

I'm really stoked, a good friend rang me last night to see if I wanted an amp chassis that he built about 40 years ago (when he was 15), otherwise it is going in the rubbish bin. I immediately said yes before he even started describing it or it's condition.

It is a 2 X 6V6GT with ECC82 PI on a chassis about 8" (200mm) square, 2" (50mm) deep and has suffered slight corrosion from being stored in a damp shed for about 8 years. It still works after not being switched on for about 30 years.

There is lots of room to add more valves for a preamp and controls if required but it's square shape make it a bit awkward to build into any excising cab design I have seen.

My initial thoughts were to build something like a Fender 57 Deluxe cabinet and put this chassis in the bottom and build a 5E3 chassis populated with just the preamp components in the top.

The other option is to build the 5E3 chassis and strip the chassis for parts. I'm not too keen on this I am sure he would like to see his handiwork put to good use.

I picked on the Fender 57 Deluxe as it seems to be well documented and uses that valve configuration, but am open to suggestions and it doesn't have to be a clone of any amp, it could be a mix of say a Marshall style cab with a Fender amp.

What do you guys think, I am open to suggestions for case style and chassis arrangement but would prefer a single 12" speaker to keep the size and weight down (and I have more old 12s than is usually considered healthy).

Thanks in advance for any suggestions.

Cub · Post by **Cub** » 18 Apr 2012, 14:33

Ah, 6V6 are my favourite tubes ! Powerful enough, lovely cleans and a great crunch.
I'd say a Marshall Studio 15 or a Trainwreck Express.

deltafred · Post by **deltafred** » 18 Apr 2012, 14:47

Cub wrote:Ah, 6V6 are my favourite tubes ! Powerful enough, lovely cleans and a great crunch.
I'd say a Marshall Studio 15 or a Trainwreck Express.

Thanks Cub, I will look those up.

I forgot to say in my OP that I will be using it mainly to play electric slide guitar through so a bit (or a lot) of crunch will be welcome.

timbo_93631 · Post by **timbo_93631** » 19 Apr 2012, 05:40

At 18watt.com there is a 18 watt lite variant the uses a 6SJ7 octal preamp tube, if you are looking for a big greasy overdriven tone that might be a nice way to go with 6V6 in the output section. Here is the thread, but you have to be a member to see it.:
http://www.18watt.com/modules.php?name= ... sc&start=0

deltafred · Post by **deltafred** » 19 Apr 2012, 09:29

timbo_93631 wrote:At 18watt.com there is a 18 watt lite variant the uses a 6SJ7 octal preamp tube, if you are looking for a big greasy overdriven tone that might be a nice way to go with 6V6 in the output section. Here is the thread, but you have to be a member to see it.:
http://www.18watt.com/modules.php?name= ... sc&start=0

Thanks Timbo, I managed to register but cannot get past the captcha to log in so am unable to view it. I will try again later.

deltafred · Post by **deltafred** » 19 Apr 2012, 11:50

I don't think 18watt want new members, I have just spent another hour trying to login with no luck. Life is way too short to waste this much time on what should be a 30 second task.

DrNomis · Post by **DrNomis** » 19 Apr 2012, 15:35

Have you considered checking out the John Mcintyre Prince O Wails Fender Princeton Mod?, I posted a PDF of the complete article in the Davidoff Library, the design uses two 6V6, two 12AX7 (I believe), and a 5U4GB rectifier tube, there's a "Soft-Clip" switch that modifies the way the two 6V6 distort when overdriven....

deltafred · Post by **deltafred** » 19 Apr 2012, 17:59

Thanks Simon I have downloaded your data and will have a good look at it, I am still wide open to suggestions.

timbo_93631 · Post by **timbo_93631** » 19 Apr 2012, 19:58

Here are some of the common 18 watt lite layouts, using the EF-86 layout with a 6sj7 instead isn't too bad, and then changing from the EL-84 to 6V6 output section can be done with a few value changes.

ef86_lite_145[1].pdf: (41.45 KiB) Downloaded 82 times

ppluis0 · Post by **ppluis0** » 20 Apr 2012, 02:01

Sorry to derail this thread, but... Timbo: What happened with the design of an 18 watts with dissimilar tubes you had suggested some time ago ?

You know: the schematic that employs one 6V6 and one 6BQ5 in the push pull stage.

Cheers,
Jose

timbo_93631 · Post by **timbo_93631** » 20 Apr 2012, 05:42

Still sitting there on the shelf in the shop. I have been busy with life (wife kids, maybe a 3rd one of those on the way!) and so many pedal builds and amp repairs, plus school, I havent set aside any time for developing it further. I have the iron and my notes and layout, but there is still some work to be done in getting it to work out. I got a MPS 6L6 PP OT in a trade recently that has got me thinking about doing an 18watt but with cathode biased 6L6's, it'll have to get in line behind the dissimilar 18.

ppluis0 · Post by **ppluis0** » 21 Apr 2012, 01:06

timbo_93631 wrote:Still sitting there on the shelf in the shop...it'll have to get in line behind the dissimilar 18.

I look forward to any news on this amp. Don`t forget to keep us posted when you resume that project.

Thank you for your answer.
Jose.

deltafred · Post by **deltafred** » 23 Apr 2012, 09:16

Well I got it and hooked it up to my Celestion G12H (in a crappy chipboard open backed cab) and ran a Strat through an SFT for a bit more gain and it sounds really sweet.

I tidied it up a bit round the power supply caps and checked them, all ok. I swapped the very short mains lead for a standard IEC chassis mounted mains connector, and the big assed 1940's ex military radio fuse holders for something a little more in keeping.

There is something not quite right going on round the feedback/presence control, it is unusable, move it off the end stop and the volume drops dramatically as does all treble. I will have to draw the schematic out.

Next job is to modify a valve preamp, that I originally built for another job, to replace the SFT.

Here is a photo my friend sent me.

DrNomis · Post by **DrNomis** » 23 Apr 2012, 09:40

deltafred wrote:Well I got it and hooked it up to my Celestion G12H (in a crappy chipboard open backed cab) and ran a Strat through an SFT for a bit more gain and it sounds really sweet.

I tidied it up a bit round the power supply caps and checked them, all ok. I swapped the very short mains lead for a standard IEC chassis mounted mains connector, and the big assed 1940's ex military radio fuse holders for something a little more in keeping.

There is something not quite right going on round the feedback/presence control, it is unusable, move it off the end stop and the volume drops dramatically as does all treble. I will have to draw the schematic out.

Next job is to modify a valve preamp, that I originally built for another job, to replace the SFT.

Here is a photo my friend sent me.

6V6GT_Amp1.jpg

Sounds like you may have inadvertently gotten the phasing of the output-transformer reversed, or the negative feedback is picking up extraneous signals from another part of the amp, also, the input of the amp could be picking up the signal from the output and your amp may be working as an oscillator rather than an amplifier at high frequencies.....

deltafred · Post by **deltafred** » 23 Apr 2012, 10:32

DrNomis wrote: Sounds like you may have inadvertently gotten the phasing of the output-transformer reversed, or the negative feedback is picking up extraneous signals from another part of the amp, also, the input of the amp could be picking up the signal from the output and your amp may be working as an oscillator rather than an amplifier at high frequencies.....

Good point about the phasing, I hadn't thought of that. I will reverse it and see what that does.

I don't think that it is caused by feedback as there is only the 2 output valves and the PI on there but it is worth a check. The input is well a way from the output and is in coax. The guy who built it was a radio ham so was well aware of pickup and feedback problems.

DrNomis · Post by **DrNomis** » 23 Apr 2012, 12:14

If that doesn't sort out the issue with the presence control I''ll be happy to have a look at the amp's schematic and see if I can work out what's going on deltafred....

deltafred · Post by **deltafred** » 23 Apr 2012, 15:44

Thanks Simon.

I have drawn a provisional schematic and round the presence control looks nothing like any of the schematics I have. I may strip it off altogether and rebuild it as a Fender or Marshall type.

I tried reversing the phase and it makes a wonderful oscillator so that is back as it was now.

deltafred · Post by **deltafred** » 25 Apr 2012, 17:43

I drew up the schematic in QUCS, printed it out, then made a few improvements then managed to loose all my work so I have scanned the printout.

I apologise for the tube symbol but that was all I had without designing my own. I think I traced it correctly but if anyone can see any glaring errors please shout up.

DS_Amp_01.pdf: (20.52 KiB) Downloaded 71 times

DrNomis · Post by **DrNomis** » 27 Apr 2012, 12:45

Hi Deltafred, just had a look at the schematic, everything looks okay to me, there's a 100nF cap going to a 50k pot, is that the presence control?, it's a configuration I've never seen before in amps, the presence control configuration I've seen is the one Marshall use in their Valve amps.....

By the way, there's nothing at all wrong with the symbols you used for the Valves.....

deltafred · Post by **deltafred** » 27 Apr 2012, 20:54

DrNomis wrote:Hi Deltafred, just had a look at the schematic, everything looks okay to me, there's a 100nF cap going to a 50k pot, is that the presence control?, it's a configuration I've never seen before in amps, the presence control configuration I've seen is the one Marshall use in their Valve amps.....

By the way, there's nothing at all wrong with the symbols you used for the Valves.....

Hi Simon

I modified the symbols so they now have a proper cathode.

I did see 2 errors, R13 and R14 should be 470k not 27k. I probably cut and pasted them from the PI stage and forgot to change the values.

Yes that is the presence control, I have never seen one like that either. It is very radical in operation almost like a switch. As you move it from the endstop it suddenly kills all treble and drops the volume to about half so is pretty useless as it is. I will try reducing the value of C8 and if that does not improve it I may just remove it completely.