4 power tubes vs 2

[Stratz]

Hi all. I've built some guitar amplifiers and converted over 30 PA/Hi-Fi amps to guitar but I've never built an amp with 4 power tubes. I'm not exactly sure how to go about wiring the power tubes and the bias will be new to me as well.
I have all of the parts for a 6AQ5 x4 build (including JAN 6005 tubes which are very tough and sound fantastic) so I'm thinking a single channel Fender Princeton AA964 (schematic attached) without the reverb and Tremolo.
I'm guessing that there's more than wiring and biasing to consider so I could really use your guidance on this one.


They're great sounding tubes and I've done 3 conversions of these amps but only with two power tubes and I love the sound but low power. I measured one of my 2x 6AQ5 builds and the wattage was only about 8 or 9 watts if I recall.
Have any of you ever built a 4x 6AQ5 amp? If so how did it sound? I'm hoping that using a quad wont change the tone much if at all.

I'd like to get started on it soon so any help would be greatly appreciated.
Thanks
Rob.

[Chuck H]

A quad of 6aq5's for a Princeton would probably be a dark sounding amp. First, you'll need to consider the voltage limits of the 6aq5's. They have a max of 250V. They'll go higher, sure, but if you're going to grind on them a lot you'll need to keep any over voltage on the tame side. Then there's "miller capacitance" that occurs when you start paralleling power tubes which increases their observed internal capacitance. Since the AA964 runs high-ish voltage on a pair of little bottles, and it can be a bright amp, something might get lost running lower voltage and four bottles. Just thinking out loud. Of course tweaks could be made post build and I would predict a creamy overdrive beast for such an amp. But if you'll be grumpy about losing some clean tone snap then don't be surprised.

[Stratz]

Hi Chuck. I am concerned about the effects of paralleling power tubes. I've heard 50 and 100 watt EL84 amps that didn't sound too great. I have two other 6BQ5 amps that have two power tubes in push/pull and the plate voltage is over 350VDC but I've been running the equivalent JAN 6005 tubes for a couple years now with no issues. The amps still sound great and after the first couple weeks the bias has been very stable.

I still need some information on what else may be needed to run the 6005's in parallel and how to bias a quad.
Thanks for the reply.
Rob.

[Chuck H]

Well, to make it more apparent, the aa964 has 420V on the rail. dropping that to 350V (which is still high for 6aq5's but "I" don't think it's a problem) will surely change the preamp voltages even if you compensate the other rail resistors. That's going to make for a "browner" sound. Add miller capacitance to that and you have a notably darker amp than an aa964.

As to four power tubes... A 100W el84 amp isn't four power tubes (I don't care WHAT Traynor ever said ). Four of any bottle should be pretty easy to manage tonally with some fine tuning after the build. IMHE the difference can be a good thing. Consider the AC30 and others. Chimey cleans and greasy leads. I built one of my 2xel84 designs as a 4xel84 as a custom order. It worked out great with only a little fine tuning. Different, yes. But still great. It's when you get into six or more power tubes that miller capacitance starts to become an issue WRT guitar amps IMHE. But there is a difference in feel between two tube and four tube amps. And I'm with you. I've always preferred the two tubes in PP sound.

[Justin Thomas]

I imagine a trip to GC to try an AC15 & AC30 might be insightful...

Personally, assuming a 10" speaker, I think Princetons are too bright, anyway... But maybe that's more a "lack of bass" through a too-small speaker. Never had a chance to try one through my 2x15.

I think a 4x6005 amp could be cool - and your note about "low power" with your other 2x6AQ5 amp makes me want to ask - what speaker, and what's the efficiency? As so often happens, speaker choice may very well be key, AND the path to "more (perceived) power."

Justin

PS: you forgot to attach the schematic!

[g1]

A 100W el84 amp isn't four power tubes (I don't care WHAT Traynor ever said ).

If they ever claimed 100W from a 4 x 6BQ5 amp that is certainly news to me. ?

[Enzo]

Classic 100 from Peavey. 100 watts from EIGHT EL84s.

[Chuck H]

If they ever claimed 100W from a 4 x 6BQ5 amp that is certainly news to me. ?

I was just baggin' on the old Traynor company for some of their (crazy, but sometimes reported as true) wattage claims. I don't know that they ever proposed such.

[g1]

Ah. The 90W from pair of EL34's I guess.

[Justin Thomas]

Hey, they didn't say at what frequency or for how long, right? So, maybe, just maybe...

Justin

[Stratz]

If they ever claimed 100W from a 4 x 6BQ5 amp that is certainly news to me. ?

They were custom built for Robin Trower by Cornell amps. He used 2 Cornell “Plexi” 18/20 amps on the Seven Moons album with Jack Bruce and the 50w and 100w amplifiers live. He didn't use them for long. I didn't care for the sound much either.
You can hear and see the Cornell's in this video. Not his best tone at all.

https://youtu.be/uSQ9ZCPEAuY

[Stratz]

I imagine a trip to GC to try an AC15 & AC30 might be insightful...

Personally, assuming a 10" speaker, I think Princetons are too bright, anyway... But maybe that's more a "lack of bass" through a too-small speaker. Never had a chance to try one through my 2x15.

I think a 4x6005 amp could be cool - and your note about "low power" with your other 2x6AQ5 amp makes me want to ask - what speaker, and what's the efficiency? As so often happens, speaker choice may very well be key, AND the path to "more (perceived) power."

Justin

PS: you forgot to attach the schematic!

I dont have a schematic Justin. I'll use the AA964 tone stack and wing it from there.

[Stratz]

As far as speakers go as mentioned above it's going to be a head only so I can play it through 1x12"/2X12' and even a 4x12" if I want to see what it can really do.

I'm still not sure if each set gets it's own bias resistor/supply or not.
I'll have to look at some Vox schematics to figure out how the power section is wired and biased.

[Stratz]

and your note about "low power" with your other 2x6AQ5 amp makes me want to ask - what speaker, and what's the efficiency? As so often happens, speaker choice may very well be key, AND the path to "more (perceived) power."

Justin

PS: you forgot to attach the schematic!

Wattage was measured through my 8 ohm dummy load Justin but when I play them through a 1X12 they seem on par with a pair of EL84's even if the 6005 is rated for less PD.

The thing I really like about the 6AQ5/6005 is that they dont fart out like the EL84's so often do at high volume.

[catalin gramada]

Think I read somewhere with 4 tubes against 2 the amp damping factor is more improved. If so more output tubes means better bass control, that.s nice.