Will Too Many Tubes In Parallel Sound Bad?

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Question:

If you want to reach at least 25 Watts with a single-ended guitar amp, that means that you have to use several tubes in parallel.(maybe 4?)

Depends on the tube. So my question is if too many tubes in parallel will sound bad. I’ve heard somewhere that you lose dynamics if you connect too many in parallel. Is that true?

Where did you hear that? I wouldn’t expect a problem, unless you used random, un/mis-matched tubes. John noted that two tubes was barely louder than one. This makes sense. Doubling the power doesn’t raise the volume much. Remember, it takes 10x the power to double the volume. Doubling the power is mostly going to be noticeable as more headroom. Quadruple the power would be more noticeable. Finally, don’t forget that any design using more than two outputs at once into a single OT is running tubes in parallel. A 4 tube output stage has two tubes in parallel on either side of the OT. Several amps, of which the Ampeg SVT is teh best known, used three on each side of teh OT. The SVT was a bass amp, but had more dynamics than most people could stand (at least, close to it without earplugs 8^). At least one amp had *four* tubes on each side of the OT. It was a big, honking Fender. I’ve never heard one that I know of, but I’d like to. Ken Gilbert’s page on his the BAGA (big a** guitar amp) has 12 KT90s – two channels of three KT90s in P-P per side, with both channels slaved together. Worth checking out just for the pic of the tubes: http://www.ken-gilbert.com/images/tubes/kt90s.jpg -Miles

Response:

If you want to reach at least 25 Watts with a single-ended guitar amp, that means that you have to use several tubes in parallel.(maybe 4?) So my question is if too many tubes in parallel will sound bad. I’ve heard somewhere that you lose dynamics if you connect too many in parallel. Is that true? /

One of my curent works in progress is a parrallel SE amp with 6L6 output. The 6L6s will give you 25 watts if you push them abit. I had mine putting out 12 each, 11 watts is the published max. Lately I have been experimenting with SE Push Pull. Its and odd output, sounds more SE then Pushpull but less noise then SE, but noisier then Push Pull. It gives abit more volume, but It does not have the grit of the plain old SE output. The hardest part of makeing SE amps that run over 15 watts is getting a good output transformer that can handle it. I tested my amp through a hammond 125ESE and it handled the extra power fairly well, but you could hear the transformer distorting when the volume was maxed. Right now I have it with a 125ESE for each tube, each feeding their own speaker, I guess this makes it more of two seperate power amps fed by the same pre, but it sounds great. I will bring it back to a full parallel SE output when I find a 30 watt SE output transformer. For awhile I had it as a Parallel 6V6 output which really sounded great, but not enough volume. So I dumped the tube rect and went solid state to give the 6L6s a little more juice. The 6l6s sound more intresting to me, but then again the 6V6s sounded just like my standard 6V6 SE amp, just a little louder, and abit more grit. Adam

Response:

My latest homebrew started out as a switchable 1 or 2-6v6 in pentode (it’s now permanent triode) single-ended design. There wasn’t much volume difference either way (1 vs. 2 tubes).

plate impedance should be matched to the load for best power transfer (thevenin). I’m not surprised there wasn’t a difference as you weren’t using the extra current capacity. or were you? did you drop the primary impedance in half on the OT when running in parallel? — [species: human; planet: earth,milkyway(western spiral arm),alpha sector]

Response:

My latest homebrew started out as a switchable 1 or 2-6v6 in pentode (it’s now permanent triode) single-ended design. There wasn’t much volume difference either way (1 vs. 2 tubes). plate impedance should be matched to the load for best power transfer (thevenin). I’m not surprised there wasn’t a difference as you weren’t using the extra current capacity. or were you? did you drop the primary impedance in half on the OT when running in parallel?

Yes.

Response:

A couple of factors, the extra tubes are a tougher load for the driver, especially at high frequencies. Stability can be an issue, but grid snubbers are almost always enough. I don’t think you’ll have a problem, prefer a 12AT7 over AX7 for the driver. You could get close enough with one SE 6550. Two or three 6L6 or EL34 would do depending on how hard you’re willing to run them. What’d you have in mind for a tranny? Ron

Response:

- Hide quoted text — Show quoted text – If you want to reach at least 25 Watts with a single-ended guitar amp, that means that you have to use several tubes in parallel.(maybe 4?) So my question is if too many tubes in parallel will sound bad. I’ve heard somewhere that you lose dynamics if you connect too many in parallel. Is that true? / One of my curent works in progress is a parrallel SE amp with 6L6 output. The 6L6s will give you 25 watts if you push them abit. I had mine putting out 12 each, 11 watts is the published max. Lately I have been experimenting with SE Push Pull. Its and odd output, sounds more SE then Pushpull but less noise then SE, but noisier then Push Pull. It gives abit more volume, but It does not have the grit of the plain old SE output. The hardest part of makeing SE amps that run over 15 watts is getting a good output transformer that can handle it. I tested my amp through a hammond 125ESE and it handled the extra power fairly well, but you could hear the transformer distorting when the volume was maxed. Right now I have it with a 125ESE for each tube, each feeding their own speaker, I guess this makes it more of two seperate power amps fed by the same pre, but it sounds great. I will bring it back to a full parallel SE output when I find a 30 watt SE output transformer. For awhile I had it as a Parallel 6V6 output which really sounded great, but not enough volume. So I dumped the tube rect and went solid state to give the 6L6s a little more juice. The 6l6s sound more intresting to me, but then again the 6V6s sounded just like my standard 6V6 SE amp, just a little louder, and abit more grit.

Fun project, what sort of pre and driver are you running? For the output tranny check out some of the DIY tube audio stuff. Some insanely expensive stuff out there, but also some very reasonable. When looking at them remember that their power ratings are based on things that don’t apply to guitar amps. The power ratings are based on triodes running buckets of current and saturating the core and frequency response down to low bass. The lowest note on a guitar is 80-some Hertz and there’s no 8 watt guitar amp that draws 80mA. What they call a 20W tranny will do 30 very, very well for us. Ron

Response:

– Hide quoted text — Show quoted text – One of my curent works in progress is a parrallel SE amp with 6L6 output. The 6L6s will give you 25 watts if you push them abit. I had mine putting out 12 each, 11 watts is the published max. Lately I have been experimenting with SE Push Pull. Its and odd output, sounds more SE then Pushpull but less noise then SE, but noisier then Push Pull. It gives abit more volume, but It does not have the grit of the plain old SE output. The hardest part of makeing SE amps that run over 15 watts is getting a good output transformer that can handle it. I tested my amp through a hammond 125ESE and it handled the extra power fairly well, but you could hear the transformer distorting when the volume was maxed. Right now I have it with a 125ESE for each tube, each feeding their own speaker, I guess this makes it more of two seperate power amps fed by the same pre, but it sounds great. I will bring it back to a full parallel SE output when I find a 30 watt SE output transformer. For awhile I had it as a Parallel 6V6 output which really sounded great, but not enough volume. So I dumped the tube rect and went solid state to give the 6L6s a little more juice. The 6l6s sound more intresting to me, but then again the 6V6s sounded just like my standard 6V6 SE amp, just a little louder, and abit more grit. Fun project, what sort of pre and driver are you running? For the output tranny check out some of the DIY tube audio stuff. Some insanely expensive stuff out there, but also some very reasonable. When looking at them remember that their power ratings are based on things that don’t apply to guitar amps. The power ratings are based on triodes running buckets of current and saturating the core and frequency response down to low bass. The lowest note on a guitar is 80-some Hertz and there’s no 8 watt guitar amp that draws 80mA. What they call a 20W tranny will do 30 very, very well for us. Ron

The preamp and driver changes alot, it started out alot like the VOX TB, and it has slowly become a nice sounding mess that gives you a headache to look at. Its running with a 6SL7 for the pre fallowed by the TB tone stack and that feeds both halves of aother 6SL7 and each half of the 6SL7 feeds its own 6L6. It changes every other week or so. Just haveing fun playing around with this amp. I take a peek into the audiophile DIY tube world everynow and then to look for a transformers and the such, but I allways end up at OTLs and trying to convince myself to buy a couple dozen 6AS7s (or are they 6SA7s?) and building a circlotron. I can wait on the output transformer untill I decide on the design anyways. But that will probably never happen. adam

Response:

So my question is if too many tubes in parallel will sound bad. I’ve heard somewhere that you lose dynamics if you connect too many in parallel. Is that true? I don’t know about losing dynamics from too many in parallel, but I will say that the only amp I have that uses a parallel pair (1952 Gibsonette) makes much less volume than any of my push-pull 2-6V6 powered amps. In fact, it is only slightly louder than my (1-6V6) Vibro-Champ.

I agree with John. My latest homebrew started out as a switchable 1 or 2-6v6 in pentode (it’s now permanent triode) single-ended design. There wasn’t much volume difference either way (1 vs. 2 tubes). I suspect I’ll try a low voltage (low power) 2-6v6 push pull next time. At least the hum and some of the noise will be lowered. I would guess a pair of EL34’s would get you pretty close to 25 watts, wouldn’t it? A pair of 6550’s would surely reach that power. I suspect you can lose dynamics (and alot more) if you don’t select the right output tranny for both DC bias and load impedance as you scale up. The right tranny will be big, heavy and expensive but if you go with the right one I would think dynamics and everything else would be just fine. Take care, stringbend

Response:

I don’t know about losing dynamics from too many in parallel, but I will say that the only amp I have that uses a parallel pair (1952 Gibsonette) makes much less volume than any of my push-pull 2-6V6 powered amps. In fact, it is only slightly louder than my (1-6V6) Vibro-Champ. Excellent question though! John King

Response:

: If you want to reach at least 25 Watts with a single-ended guitar amp, : that means that you have to use several tubes in parallel.(maybe 4?) No, not really. But if you use the most common designs and the most common tubes, you will probably parallel smaller tubes for increased power. It can be done with one tube…. just not common on the consumer electronics maket and expensive to make. : So my question is if too many tubes in parallel will sound bad. : I’ve heard somewhere that you lose dynamics if you connect too many in : parallel. Is that true? Tube C values sum in some designs. cheers skipp

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