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gkenne 31st January 2013 09:50 AM

How to add an additional pre-amp stage to 5A3 design
Okay, so I have built a 5A3 amp from and old 1953 Bell Sound Labs 2122-B integrated amp. Original configuration has two 6SC7's, one 6SL7, two 6V6's and a 5Y3GT rectifier. I couldn't really figure out how to get both of the 6sc7's into the build. I have seen that the Bassman has the same number of tubes but, it also has a different two rectifier setup and no doubt heavier transformer for those four speakers. So, my question is this, how would I best include the second 6SC7, into it and hopefully get a few more watts of usable power. I would think it wouldn't be all that hard to do. I am only using one input, versus two separate input jacks. Any ideas welcomed. Thanks in advance, once again.

gkenne 5th February 2013 11:53 PM

anyone help me with this: I have my Fender Deluxe 5A3 build. The platform has another tube (octal) 6SC7 available, I want to add this additional tube as another gain stage. I'm just not sure of the best way to connect this into the current design, I have been looking at the Bassman 5E6-A and the Super-Amp 5E4-A, both have three tubes like that in front of the output tubes. Should I leave the volume, bass and treble where I have it and just move the actual input to the additional tube, then add another master volume, or how would I make it a pure gain stage, gain control stage?? PS: I only have one input versus the multiple inputs on most of the Fender schematics.

tinitus 6th February 2013 12:14 AM

many questions in one post tend to make it difficult and confusing

gingertube 6th February 2013 02:55 AM

My advice - possibly worth what you paid for it.

First, you will not get any more power from any scheme to utilize the other 6SC7. the only Power amp change which could be contemplated is rerwiring of the phase splitter from a para-phase to a differential splitter.
Soome folk like the sound of the paraphase splitter, some prefer the differential splitter (I use a 3rd option the Concertina Splitter on my PP 6V6 Guitar Amp).

What you can do is have a more sophisticated preamp section.

One option is to use a Trainwreck preamp - see the A0 circuit here.
Just use a 6SC7 triode section in place of each 12AX7 triode section, circuit values around the tubes can stay the same.
That gives you what is basically a "Black Face Fender" with an additional gain stage. That cold biased gain stage (last stage) with the 10K cathode resistor is known to add some pleasant overdrive tones.
You need to find space for 4 controls B,M,T and Volume.

There are many other options including a 4 triode version of the above. If interested in that, do a search for "Junkbox Trainwreck" by yours truely (Gingertube). It is that A0 circuit with parallel triodes in the first stage.

If you have any of Kevin O'Connor's TUT Series books then 4 triodes ( 2 off 6SC7) is enough to do a Clean / Lead, 2 channel preamp which can even be footswitched if you want - LPSP "london Power Standard Preamp" is what you would be looking for. My own guitar amp uses this preamp design with 6SL7s inplace of 12AX7s. In this case you need to find space for 8 controls, Gain, B, M, T, Volume for Lead, B, M, T, Volume for Clean and a Master Volume.

You could also copy any of the standard Marshall etc. preamps (but I would'nt, if you like Marshall Lead tones then that London Power LPSP will give you Fender Clean and Marshall Lead tones).


gkenne 7th February 2013 12:34 AM

Thank you very much, for your input. Any bit of info. helps me on this quest of mine. I did look over your Junkbox Trainwreck design. I will consider this as it does have three tubes before the output tubes. One other question I have. My build currently has 2 6V6's in the output, what if I were to replace them with 6L6's, which would go from 12Watt output to aprox. 20 Watts. Would I, in your opinion gain actual output from them, (they are the same pinouts) or am I only limited by the output transformer's limits. Would there be any other problems I might have by doing that?? Thanks again, in advance.

gingertube 7th February 2013 02:44 AM

I would not contemplate putting in 6L6. You said that the original configuration was 2 x 6V6 and a 5Y3 rectifier. That indicates to me that the power supply would not cope with 6L6:
- the power transformer would not provide enough heater current
- the power transformer would not provide enough High voltage current
- and even if it could provide the high voltage current the 5Y3 would not handle it
The output transformer would not be suitable in that:
- it would not handle the extra output power
- to a lesser degree, it's primary impedance would'nt match.

Stick with the 6V6 and the 5Y3, there are many highly regarded amplifiers which used this combination, Fenders in particular. I don't know why you would need more power than this unless you are a real shreader metal head type player.

If you really need more sound pressure then look at changing speakers instead. More cone area shifts more air which gives you more sound pressure. More cone area can be achieved by using bigger speakers or more of them or both.


gkenne 7th February 2013 06:26 AM

Yeah, I did a little more research about the 6L6. I won't be doing that. But, I might eventually do something like what you have going with the the Junkbox Trainwreck. The next thing I'll probably do is build a more suitable cabinet. The speaker I have is one from a Fender twin. I think it is a pretty good one for the application, since the original Deluxe's came stock with a single 12". A nice pine cabinet, or a separate cab, head units will probably have a better sound than what I'm using. Right now it's in a speaker box from a Yamaha system, home stereo, the pressed fiber material is very dead and probably gives it a deadened sound. From what I'm reading, one of the things that gave Fender, Marshall and Gibson their great sounds was the cabinets themselves. Thanks again. Something that I did do was a couple of the items mentioned in this project ed=0CD8QFjAB&url=http%3A%2F%2Fcourses.physics.illi ames_Milsk_Gabe_Jacome%2FJames_Milsk_Gabe_Jacome_M odified_5E3_Deluxe.pdf&ei=3kcTUZfNLMOQ2AXI7YH4DQ&u sg=AFQjCNEk5vX5ISVFhl_I67dufGu9JgKC2g&sig2=n7Jthn-iPzQSQoO7HnMHiw&bvm=bv.42080656,d.b2I

I did a couple of the changes to mine that sounded like they made sense, I would like to know what you think of them, I added a negative feedback loop, it does give a bit more headroom, but whistles in the off position, so I have to start the machine with it engaged, I also tried to hard wire it in place but also got the high pitch whistle. Couldn't figure out what I was doing wrong. So, I just decided to leave it on. Once it's warmed up I can turn it off and it lowers the volume a little. Thank again. Jerry

gkenne 16th February 2013 01:43 AM

Well here's the latest in the saga. I tried to do a modification to the 5E4-A and I have no power at all, barely can hear it. It is very quiet though, so my start grounding attempts seemed to have paid off. The 6V6'S only have aprox. 310Volts to them, not near as much as the Super or the Bassman. Well I guess it's back to the drawing board. I suppose unless I upgrade the PT and the Output Transformers I can forget about getting any additional gain by using the additional 6SC7. Just as you tried to tell me. I couldn't quite figure out how to use the Junkbox trainwreck modified to work with the 6SC7's instead of the 12AX7's. Right now I basically have the first two 6SC7'S setup just like the 5E4-A Super-Amp, the 6SL7 that's the PI is setup still just like the Deluxe 5A3, the only thing else I did was took out the 25-25 Cap and the 270 Ohm Resistor on the cathode. Thinking if I reduced those I would get more power from the 6V6's. Well doesn't look like that worked, so I think I'll take it back to the Deluxe setup and call this little experiment good. Another question, why is it that it has no power? Is it because the 2nd pre-amp tube is sharing the voltage with the first one, reducing the amount of voltage required to drive the signal. Thanks talk to you soon, I hope.

ivan H 23rd February 2013 02:51 AM

Hi, ok just going back a little, u said u added a negative feedback loop & it whistles. It sound like its actually taking positive feedback, so u can either swap the OT's plate leads at the tube sockets, or u can swap the leads supplying signal to the control grid of each power tube. These can be swapped at the power tubes or the outputs of the phase inverter. Ok, here's at tip that'll serve u well, when modifying a circuit (or anything for that matter) only do 1 mod at a time & then test thoroughly. That way, if something aint right, u know where the problem lies. Now u said it was working fine, then u played with the phase inverter looking for more output, yes? Well the phase inverter is a pretty important part of the power amp. Unless u really understand how it does what it does, dont mess with it, especially if its working like its meant to. Put it back how it was. Now to adding a gain stage. I'm not familiar with your deluxe schematic off the top of my head (marshall freak), I'll have a look later today when I get home, but u havent said wot u want the added stage to do, really. Do u want to maintain the pre amps basic tonal characteristics with just a slight extra "push", do u want it to add a little overdrive "crunch", or do u want it to clip & distort heavily. Either way u will most likely need to tweak to taste befor its exactly how u want, but as was said b4, adding a pre amp gain stage will not yield more wattage. There is a wealth of information out there if u want to learn about how these circuits work. Cheers

gkenne 23rd February 2013 03:12 AM

Yes, thanks again. I am learning quite a lot here. I guess what happened is that when I tried to do the change to the Super-Amp type, it uses an entirely different style of PI, I got it to work like I said, but I got cold feet. Especially since I had it working pretty well before, I suppose I would have had to change just about the whole amp. I started thinking about that the Super and Bassman both have more power to start out with and the Super has 2, 12AY7's, a 12AX7 and the two 6V6's, with a 5U4GA. That tells me that is has a bigger transformer. So, I don't think mine has enough power to push. I don't really know for sure. The Bassman has, the same pre-amp and PI as the Super, but has two 5U4GA's to push those four 10"s. I think I'm just going to leave it as is right now. The last thing I did was to link the two inputs, as if they were jumpered and it completely changed the amp, much more control, with the second volume giving it much more gain. I think I have it sounding pretty darn good right now, it's fairly loud. With some break up. It does make kind of a feedback type howl at full volume, but is okay backed off just a little. The next thing would to be building an appropriate cabinet for it and maybe use the 6SCGT's instead of the metal cased ones, I've heard they have less microphonics problems. I'm leaning more toward just building something else by kit next. I will continue to learn more, I don't really understand all the technical stuff about, load lines and all. Starting to get a pretty good idea of how to bias though, and how that affects the amp. Some of it's still a little confusing. Alright thanks, take care.

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