Chosing a power transformer

[Bluefinger]

Hi,

I am planning to build a harp amp using a 2x6L6 fixed bias power section and two 12AX7 (12AY7?) preamp tubes, including the PI. It should be something like a 5F4 PI and power section with a Blackface Princeton tone stack and V1. Filtering will most likely be rather tight and chances are that I will use a GZ34 or solid state rectifyer for less sag. For good low end punch and efficiency I might try my luck with a Super Reverb/Pro Reverb output tranny. This is the rough plan but things might change during the build and testing.

The options seem to be endless. I am not sure about plate voltages I am going to use as this will be determined during the build, depending on how it sounds. Is there a safe bet? I assume a 5F4 power transformer would be ok as I can always drop voltages. WHat would you think? Any other suggestions?

thanks!

[MWJB]

What speaker config? If a 2x10" I'd aim to stick <470vdc, 4X10" will be happy with 470vdc up to 530vdc. This equates to 345-0-345 to 355-0-355VAC PT for the former, anything up to 375-0-375 to 385-0-385VAC for the latter. The very high B+ will lead to an amp that is very loud & punchy, the 470-ish amp will be a bit more flexible at smaller venues.

A typical 5F6A bassman PT produces 470vdc at 30-35mA with a GZ34. A 5U4G might drop 20v from that. Running cold bias you might see 485-490vdc with a SS rectifier. This is the PT was used in the brown Super.

A Brown Concert style PT (SS rectified)/higher voltage Super Reverb PT will be good for the latter BUT CHECK specs with the mfr/vendor in person, there are lots of options, all calling themselves the same thing, but many run differing B+. If you are building from scratch and use a stand up PT the Hammond 374BX (375-0-375) will give 510vdc, SS rectified at cooler bias (it fits the Concert/Twin footprint as lay down but primaries & secondaries exit from different sides of the PT, rather than Fender style all out the inboard side).

Minimum B+ I would entertain for 6L6 in fixed bias is around the 440v mark. PT B+ specs are "loaded" (Mercury & some other mfrs unhelpfully list the unloaded spec. which is confusing!).

3x10" will be reasonably happy with either B+ and could even calm down the high B+ amp, as could 1x15".

I wouldn't, personally, go for the 5F4A tweed style cathodyne PI w/presence control, I'd go for the late 5F6A/6G12A PI/presence as described in the '59 RI/6G12A schem (not original Fender 5F6A schem) and replace the 6.8K tail resistor with 10K.

[Bluefinger]

The planned speaker configuration is 1x10 + 2x8. I might try try 1x12 + 2x8 but I don't think that I'll want anything bigger than 10s in this one. I always liked my 5F4 or 5E5A better for harp than the RI Bassmans that many players are using. The Tweed Super/Pro/Bandmaster circuit sounds more old school to me. The Bassmans are too punchy and in-your-face for my taste sometimes, lacking that certain sweetness. That power section but with less gain stages in the preamp and a full TMB tone stack is what I would like to try. I have no idea if it's going to be practical but the research is half the fun, isn't it?

That Hammond tranny looks good. I think I'll also check with Tube Amp Doctor, simply because it might be cheaper to buy within the EU and I never had problems with their products. I don't know, they might be relabeled Heyboers, which wouldn't be bad thing at all. Thanks to your response, I now know what I should be looking for.

Thanks a lot!!!

[MWJB]

Well, you seem to have a good idea of what you want, I would also point out that the 5E# amps have a different preamp circuit too...maybe a 5E# preamp mated to the 6G12A PI...just a thought...? Means another preamp tube though. The tolex preamp with 12AX7s seems to end up about the same overall gain as the later tweed with a 12AY7 in V1. In effect, it is only 2 preamp stages plus PI, just like the Normal channel tolex.

If you wire the first filter cap to the hot side of the standby, like a BF amp, rather than original tweed, you can use a 5R4 rectifier for more sag, but 5U4 can sweeten up a 5F6A, I know a few players who use 5U4 in a 5F6A and get a sweet sound. A few guys have recommended the Eminence Cannibis Rex but good harp choices in 12", even in 8" are perhaps more limited than 10".

The TAD made in USA transformers are the same as supplied by Mojotone, so probably Hyboer? Allparts in the UK is also a Mojotone dealer, stock can be patchy though.

[Bluefinger]

What would be the benefit of using the 6G12A PI over the 5F4 type? I am ready to do some research anyway and to try a couple of things like different PIs and tone stacks but I have to start with something.

I just found a german supplier that has everything you can ask for including Hammond transformers for less than what the Heyboers cost. I will probably get some stuff there.

thx!

[MWJB]

Although the PI of the 5E# tweed is deemed to have no gain, it always has a preceeding, unattenuated stage, meaning that the late Bassman/Concert LTPI amplifies by a factor of around x15 (depending on B+) but the 5E# style cathodyne PI amplifies by 40-50 times...that's quite a lot of gain that you don't really need. The presence control works better too.

If you find the bassman/concert style too tight, going bigger on the tail resistor can loosen things up, I find 22K a bit too wallowy, but if a looser sound is what you are after it might work for you.

[txstrat]

...maybe a 5E# preamp mated to the 6G12A PI...just a thought...?

That's what I've done (well, actually a 5F4 preamp with a 6G11 PI - no big difference) and it's my main amp ever since I've heard it. Great sweetest tweed super tone with later breakup.
Highly recommend it.

BTW I'm using two Jensens C10Q

[Bluefinger]

ok, I see. Less gain is mostly a good thing in a harp amp. I'll give it a try ...

thanks!

[Bluefinger]

On the other hand .. I could simply skip the first half of the cathodyne PI and leave it unused as only the second half acts as a PI ... I don't need a presence controll anyway. I will probably end up trying a couple of different combinations ...

[MWJB]

Well, this might work with 6L6 & higher gain/B+, but remember that this is how the non-reverb Princeton is wired (input triode>gain stage triode>PI triode), subsequently they have low gain, even for harp, some guys love them for this, some don't.

[Bluefinger]

That was my initial idea behind it ... so I can add that additional gain stage (first half of V2) depending on how much gain I want. I could for example also try to put a 12AY7 in V1, so I would have two rather tame 12AY7 gain stages in series with the tone stack in between and then that strong one on V2 before the PI. For guitar anything weaker than a 12AX7 in V1 of a Blackface amp usually is too much signal loss and the overall volume suffers but maybe it works with the strong input signal of a harp mic ... I don't know ...

[MWJB]

"For guitar anything weaker than a 12AX7 in V1 of a Blackface amp usually is too much signal loss and the overall volume suffers but maybe it works with the strong input signal of a harp mic ... I don't know" For some players a 12AY can work great, but most guys who have tolex Fenders run 12AX7. I have an amp that basically has one triode feeding a LTPI, gain is way too low for guitar, even too low for a typical harp mic unless you push the B+ (runs 6L6 at 510v), at low B+ it just sounds loose & mushy. It's in a 2 channel amp with regular layout on the other channel...they both kick out the same level of sound, just at different volume settings.

So, a single triode driving the cathodyne PI, plus a gain stage could work, plus if the input triode was just one half of a 9 pin socket you could sub in different tubes and just affect one triode at a time...this is what makes the tweeds a bit more flexible with preamp tubes.

[Bluefinger]

It will be a one channel amp since there is no use in having something like a "bright" channel on a harp amp. I don't like the idea of having the input triode and the first gain stage on the same triode either. Maybe I'll just add another tube and use just one half of either one. I really don't know ... so many options ... I guess there's no way around trying it all