Other Amps

Small tube amp comparison, but not that kind of comparison

1

I'm looking for a tube amp lesson from you guys.

So I just bought a 15 watt Fender Blues Jr IV on Reverb. It's outfitted with a 12" speaker, and comes with 3 x 12AX7s and 2 x EL84s.

I had also considered, and might get one day a 15 watt Supro Blues King that also sports a 12" speaker yet it comes with much less tubage: 1 x 12AX7 and 1 x 6L6. They are both in a similar price range.

I'm not seeking which is better by any means, I'm just curious to know what the difference is in any two amps that are so close in wattage yet so different in the number of tubes being used ( I know number of tubes does not equal wattage). How the additional (or less) tubes affects the overall sound and tone.

So what would a typical amp with 3 x 12AX7s and 2 x EL84s tubes compare to one with 1 x 12AX7 and 1 x 6L6 tubes? Other things (wattage, size of speaker) being fairly the same.

I hope this makes sense.

2

The power is coming from the power tubes, so the 6L6 on the one amp and the EL84s on the other amp.

The 6L6 is running single ended, there is only one of them.

The EL84s are probably push-pull.

A 6L6 can handle more power, but tubes in push-pull can handle more than twice the power they can single-ended. A 50 watt Bassman uses 2x6L6.

Often an amp will be rated for power by how large the transformer is.

You can run tubes conservatively or pretty hard. 15 Watts seems like a lot for one 6L6, but not too hard for 2xEL84s.

1 12AX7 is pretty simple preamp.

You usually need an extra tube section or two to work as a phase inverter when running 2 power tubes. But there is enough tubes with 3 12AX7s to have a second channel or some type of effect.

So the Blues Jr is using 1/2 12AX7 at the input, some transistor for some type of fat tone shaping, 1/2 12AX7 after the tone controls, 1/2 12AX7 for what would usually be reverb recovery, but is for treble tone, then it uses an opamp for the reverb part of the signal, then a 12AX7 for a phase splitter. That leaves 1/2 12AX7, which is at the bottom of the schematic, for ??

It has pretty typical Fender flow to the schematic, everything mostly where you expect it.

3

If you can’t find schematics for those specific amps, look at a Fender Champ schematic, and an amp with two power tubes and reverb, and even if you can’t fully read a schematic, you can probably get some sense of what’s going on/how those tubes are being used.

4

I played thru one of those Supros a few weeks ago. Pretty cool, lots of crunch.

The biggest difference you’ll hear is when the amps break up. A single ended 6L6 can produce the same power as a pair of 6BQ5’s, but a single cathode biased 6L6 will have different harmonics than a push pull pair of fixed bias 6BQ5’s. Equivalent to the sonic difference between a Tweed Champ and a Tweed Deluxe.

5

I played thru one of those Supros a few weeks ago. Pretty cool, lots of crunch.

The biggest difference you’ll hear is when the amps break up. A single ended 6L6 can produce the same power as a pair of 6BQ5’s, but a single cathode biased 6L6 will have different harmonics than a push pull pair of fixed bias 6BQ5’s. Equivalent to the sonic difference between a Tweed Champ and a Tweed Deluxe.

– Powdog

Thank you for calling them 6BQ5s like God intended just like 6CA7 instead of EL 34 for US made amps.

While I'm here, how many clean watts can a single 6L6 do?

6

At a plate voltage of 500v, max anode dissipation is 30 watts. So 15 watts is about the most a single 6L6 can do.

7

Thanks everyone, this all makes sense, even the parts I struggle to understand

I played the Supro Blues King and liked it, but when I played that Peeless P2 through a Blues JR IV I had to buy the Peerless, and now I have the Blues Jr which very well might be the best sounding amp I have ever owned.

So obviously comparing the tube set up on both amps made me curious about them. Thanks for the lesson(s)!

8

Push-Pull amps sound better because they cancel out even-order harmonics. Single ended 6L6 amps in particular require a fair amount of negative feedback just to tame the even-order harmonics to a reasonable level, although you can't eliminate them entirely in a single ended amp. A perfectly balanced push-pull amp will sound much better than a stock production Push-Pull amp. How many preamp tubes you have is not a matter of tone, but of what extra features the amp has.

9

A layperson's question, for sure, but: what needs to be done to the circuit of one's push-pull amp to get it "perfectly balanced?"

10

A layperson's question, for sure, but: what needs to be done to the circuit of one's push-pull amp to get it "perfectly balanced?"

– Lacking Talent

You have to take it to somebody like Billy Zoom.

12

I had a few Blues JR's, Got and kept the Supro. The sound is much more pure and super fantastic!

13

Sometimes it's just showing off. 'Reverse the polarity on the megnatronic rear thrust combobulator'. Actually, Buzz Aldrin fixed a circuit breaker on the Apollo 11 mission with a felt tip pen and saved the universe. I coulda done that.

14

I had a few Blues JR's, Got and kept the Supro. The sound is much more pure and super fantastic!

– Mark Kunie

Which Supro, may I ask? I have a Model 24 copy made by Cave Valley Amps and it's a solid fellah

15

You have to take it to somebody like Billy Zoom.

– Billy Zoom

Love it! LOL. Seriously Billy has been the only guy who’s worked on my amps. Great tech.

16

I believe that Supro Blues King uses tubes for the amplification and transistors for the dirt. That's fine by me if it sounds good. So many guys play tube amps with a tubescreamer for dirt and rave about the sound of tubes.

I'd love to try the Blues King - I've heard great things of it. I've had a Blues Junior and it was not for me I'm afraid. I have been much more impressed with the Bass Breaker Fenders.


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