Liverpool Circuit - Princeton Chassis - 7 pin output tubes

[RJ Guitars]

Hey guys,

I have an idea for a Liverpool circuit in a Princeton chassis with some 7 pin output tubes. This should produce a great little combo amp with a yield that could vary from 6 to 12 watts, dependent upon which output tubes you chose. I like the Princeton chassis idea for many reasons but primarily because it means that I don't have to design or build a cabinet.

Rather than prove and refine the idea, in this case I'll throw it out from the early stages and the offer for other folks build along with me (or independently) if they have such an inclination. As I often do I'll offer the parts at cost for the first dozen that share my curiosity. I won't offer any custom chassis or circuit board designs, maybe some improvements along the way as needed though.

If nobody cares for the idea personally I hope you'll indulge my curiosity (hopefully with your kind silence).

Thanks,

rj

Disclaimer #1 - This is an idea based upon a Trainwreck Liverpool circuit - it may not be a good idea and it might not work out all that well. There are many risks with any new design but a high gain Trainwreck circuit in this chassis, different output tubes, plus being used as a combo amp will likely bring some challenges.

Discalimer #2 - Prototype amp builds are generally a back burner project for me and my pace may be painfully slow for some.

[RJ Guitars]

Obviously not such an interesting idea based upon the lack of any comment from any other person over a very long period of time. However, I actually finished this up... to some degree anyway.

It works and as you might have guessed and I myself expected, it doesn't really sound terrific. I broke almost all the rules and known standards for building a high gain Trainwreck circuit. I built it this way knowing that I'd have a job ahead to tame it into submission. If I can conquer it then this layout lends itself to developing a PCB version. I still need to finish up the presence circuit and maybe that will make a significant difference in smoothing things out.

I have 240 volts DC into the output transformer and drop down to about 140 volts on the plate of V1a. It does the Liverpool attitude thing very well but is harsh sounding and doesn't have much clean headroom at all. I am currently using 6AQ5 output tubes and it is not especially loud. I haven't done the math but from the volume I'd guess 7 watts plus or minus 3 watts.

Ken Fischer gave a long discourse about the wire he used in the Trainwrecks and declared that the wire is critical part of his circuit and that the amps would sound harsh if you used teflon insulated wire. In that article he talks about the importance of selecting different components for the Komet amps to accommodate the teflon wire he used there. One of my challenges is to sort out what to change to take the harshness out of this amp because as you may have noticed or expected, I used teflon coated wire.

What I gather from the wire article is that the wire itself has electrical properties that become part of the circuit. I would expect this to actually be capacitance, but I'm still open to learning something. This notion that the wire acts as a capacitive component would imply that the location and geometry of the wire becomes critical to get the circuit to work right. By the way these are not new thoughts nor entirely my own thoughts, it's been said before. I have not yet seen a report on any one measuring it or swapping amp parts to overcome it, but I suspect that has been done as well.

On top of everything else, I wanted to use some smaller output tubes. I love to explore output tube options and there are a ton of cool 7 pin tubes to try. The output transformer has a huge primary impedance range so I can really explore a ton. For those not familiar with the 6AQ5 it is a miniature version of the 6V6 tubes. So this amp might be expected to behave and sound sorta like a cathode biased 6V6 Express... I dunno whether to expect that or not yet.

enjoy,

rj

[Colossal]

Killer layout RJ, very nice and imaginitve. I think "the rules" can suck it and when we box our selves in with self-imposed limitations, we stop growing. I doubt the harshness is due specifically to teflon wire and your layout is almost verbatim Trainwreck. I see you used polystyrenes for the treble and fizz caps. What do you think about rerouting the plate leads for the output tubes? Have you tried a different output impedance?

[M Fowler]

rj,

I was wondering what happened to that build, looks good.

So many different amps to build and not much time or money.

Mark

[JamesHealey]

I went from Teflon wire to PVC wire in an express build and the change in tone was big enough to be worth the effort, just made the amp sound less aggressive in the top end.

[M Fowler]

I have only used PVC in my Trainwrecks but some are 20 awg solid and some are stranded 20 awg, any difference in using solid verses stranded to anyone ears?

Mark

[Colossal]

I would also add that while a 125E is a versatile OT and great for experimentation and proof of concept it is by no means a great sounding OT.

[TheGimp]

I replaced the OTs in a Monarch Console stereo amp (using 12AB5s) with Edcor XPP10-8-10K and found them to be quite good for a stereo amp. The 12AB5s are similar to the 6AQ5s, although designed for lower plate voltage.

I would expect them (Edcor XPP10-8-10K) to fare well in a guitar amp, although there is the drawback that they only have one output impedance.

Nice looking amp.

[RJ Guitars]

Thanks for the good words guys.

I hadn't thought about a different output impedance for the 6AQ5's but I wanted to try some EL-95's and they are way higher impedance so I was going to make a change for them. Actually after I thought about your comment I realized I was 100% off on my impedance matching right now. I have a 4 ohm cabinet hooked up and the tranny is wired for 8 ohms... so something to take care of there.

My V1 tube was not a good choice. It was in a drawer of old 12AX7's that I save for new amp builds. It is badly microphonic and needs changed before I evaluate any further.

I suspect that I have more gain than I need to feed these little output tubes and was considering ways to back this off and still keep it Wreck sounding. I got a good idea from Zippy today for using a 12AY7 in V2. He suggested using a 12AY7 and then running the tube in parallel. You get back some of the lost gain by paralleling plus some noise canceling benefits. The 12AY7 can be a great sounding tube and there is no shortage of gain in this unit.

Time for some workbench effort.

rj

[Zippy]

I went from Teflon wire to PVC wire in an express build and the change in tone was big enough to be worth the effort, just made the amp sound less aggressive in the top end.

It would have been interesting to do that incrementally, one wire at a time, starting at the input. If there is any difference, I would expect it at the longer, higher voltage leads, early in the chain. Since I correlate the material difference to lead-to-chassis capacitance, the difference would be minimized by lifting both types of wire away from the chassis.

If it is a capacitance effect, a few prudently placed picofarad caps should be able to similarly tame the harshness. Snubber caps - ala the Dumble HRMs - come to mind.

That is also the one effect due to solid core wire - being able to lay the wire down against the chassis and keep it there.

Or it could be the color...

P.S. You're hanging it out there, RJ. I think you're going to have some cool results with this. I just hope you get it done before the Wreck Police come back from vacation.

[M Fowler]

If the wires are not Columbia red your going to have serious problems

[Colossal]

If it is a capacitance effect, a few prudently placed picofarad caps should be able to similarly tame the harshness. Snubber caps - ala the Dumble HRMs - come to mind.

Zippy, I was mulling over this last night and was wondering about the use of snubbers as a solution. Very interesting idea.

Or it could be the color...

If the wires are not Columbia red your going to have serious problems

I think you guys are onto something

[M Fowler]

I am just thinking out loud if I may.

What if you use some of the smaller tubes usually found in the All-American five radio category. Tino Z. has some guitar amp builds in his book using the smaller preamp tubes for champ and other builds.

Less gain and perhaps more compatable with the 7 pin version power tubes your currently using rj.

Mark

[fishy]

I went from Teflon wire to PVC wire in an express build and the change in tone was big enough to be worth the effort, just made the amp sound less aggressive in the top end.

My first Liverpool build used teflon solid core wire and I had some serious issues taming the beast. I found it difficult to use and although it responds well to a soldering iron , I was sufficiently put off by it to never to use it again.
I too rewired the preamp with solid core PVC and it was worth the effort too. Last time I played that amp (don't have it anymore) , it was still a tad "lively" in V1 but I actually think the ceramic socket is the main reason for that now and initially, it was probably a combination of socket and lead dress. I am not so sure it was the wire anymore although I avoid teflon wire preferring stranded PVC these days......any color though

[RJ Guitars]

I went from Teflon wire to PVC wire in an express build and the change in tone was big enough to be worth the effort, just made the amp sound less aggressive in the top end.

My first Liverpool build used teflon solid core wire and I had some serious issues taming the beast. I found it difficult to use and although it responds well to a soldering iron , I was sufficiently put off by it to never to use it again.
I too rewired the preamp with solid core PVC and it was worth the effort too. Last time I played that amp (don't have it anymore) , it was still a tad "lively" in V1 but I actually think the ceramic socket is the main reason for that now and initially, it was probably a combination of socket and lead dress. I am not so sure it was the wire anymore although I avoid teflon wire preferring stranded PVC these days......any color though

Fishy,

You've added a good piece of info there. If indeed the positive impact was all due to changes in the preamp wiring, then that gives a good spot to start the taming process.

If Zippy's thought that the PVC wire is adding some capacitive coupling to ground, then some intentional capacitance to ground on the preamp wiring would be worth experimenting with.

FWIW - I put a couple different 5157 tubes in the V2 spot and the amp started to sound better but I still had to much microphonic activity and need to keep looking for the a better tube for that spot.

After I return home from my Holiday visit I'll get back to this project and try out these ideas.

rj