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Stupid noobie question about amps with multiple power tubes?

Paul86

Member
Messages
1,504
Hello TGP
Can anybody explain to me, in simple layman terms, how can Komet amps operate with multiple power tubes? Special transformers?
Would it be a feasible mod to an existing amp - say, an old 800?

Also, does anybody here have first-hand experience with this kind of amp?

Thanks
 

Blue Strat

Member
Messages
30,077
Transformers that are spec'd to handle multiple power tubes, output transformer capable of high power and with multiple output taps, power tube sockets that are wired such as not to prevent any octal power tube from working, bias pot and power supply that will cover bias ranges of all octal power tubes...

A JCM 800 can handle El34s and 6L6GCs, at least. Also possibly 6550s, KT88s and KT66s.
 

Paul86

Member
Messages
1,504
Thanks Mike
But in the case of the JCM 800 you can't just 'plug and play', right? I mean, you have to have minimal tech skills (not my case) to putz around the internals of the amp?
 

V2

Member
Messages
641
Cathode-biased amps that can handle the current draw of the tubes in question are self-adjusting. For fixed-bias amps, like the JCM800, you need to set the bias for new tubes. This is done internally, so have a qualified person do the job. Some amps have external bias jacks and pots so that the bias can be adjusted without pulling out the chassis.
 

Blue Strat

Member
Messages
30,077
Cathode-biased amps that can handle the current draw of the tubes in question are self-adjusting. For fixed-bias amps, like the JCM800, you need to set the bias for new tubes. This is done internally, so have a qualified person do the job. Some amps have external bias jacks and pots so that the bias can be adjusted without pulling out the chassis.

+1. This is at least part of the answer you're looking for...if not all.
 

drbob1

Gold Supporting Member
Messages
27,177
Many tube amplifiers are able to be fairly easily biased using a pot (variable resistor) inside the chassis. However, most of them are not able to accommodate a wide enough range to use (in the case of the JCM800 for example) an EL34 and a 6550 without slight modifications (basically just setting the bias range), so you can "mod" a 6550 JCM800 to tolerate EL34s, for example. The case of a cathode biased amp is completely different, since the bias is determined by the interaction between the tube and the bias resistor and can be set to be safe with almost any octal tube as Blue STrat has mentioned.

Are you thinking of modding your JCM800 or did you just want to hear what different power tubes do? If the latter, there are two amps you should look into. The Emery Superbaby is a super stripped down, cathode biased single ended amp that is really good at showing what difference a power tube makes. The Palmer Drei has 3 single ended power tubes you can blend together (EL84, 6V6, 6L6) or use separately. Most larger, push pull amps, the circuit makes more difference to the amp's sound than the power tubes (hence why people still loved Marshalls even after the switch to 6550s in the 70s).
 

Paul86

Member
Messages
1,504
Thanks guys. TGP always delivering the goods.
Dr Bob, what I really want is to have an amp that allows me to use different output tubes, and I hope different sounds & feel.
I have heard clips of Komet amps, but none address the different tubes issue.
So, if anyone here has first-hand experience of how the Komet sounds with different tubes, please chime in.
 

amphog

Member
Messages
4,102
I do not have experience with the Comet, but I have a Allen Brown Sugar that will take 6v6,6l6 and el34's. There is not much tonal difference, but the point where the amp breaks up is different.
 
Messages
4,877
good comments above. I only have an observation about that Emery Superbaby. I had occasion to take one of those apart while doing some repair work on the cab. Very well built little amp....and it sounded like a thin fart. When I hooked it up to a decent speaker, that little amp came to life. The modern Jensen C10 that came in that amp was not worth the electricity it took to drive it, ime and imho.
Fwiw...on the cab. Taht mahogany facade on the front is qutie attractive...on epiece of mahogany. The problem is that in order to get a piece wide enough to build that facade/faceplate, the grain has to be oriented horizontally across the front of the amp. This almost guarantees that at some point in the amp's life those vertical bars across the speaker opening will break....it is the nature of the wood. I did a clever repair that should hold the two that had broken in this amp, but the other bar or bars will do the same thing on down the line.
Back to the OP's thread.....ime, changing power tubes in these amps is not as much of a change to the sonics as many would think. The circuit sets the parameters of the signal. Different tubes behave in slightly different ways, but it is not like you get a different amp just by changing tubes. But then....subtle differences sometimes are all we are looking for, right?
 

drbob1

Gold Supporting Member
Messages
27,177
You must be thinking of the combo, mine is a Superbaby Head, and has mahogany end caps but nothing on the front. I'm running into 1/2 of a 412 cab (the other half is driven by my London Power Session) with Weber Blue Dog 12s and it sounds great.
 

Paul86

Member
Messages
1,504
Exactly Steppin' Wolfe. There is one video of a Morgan Amp where the guy tries out different tubes. The differences, if there are any really, are very subtle. But I think it's very hard judging gear from what you hear on YouTube clips. The best test, imo, is plugging into an amp and hearing things first hand. And sometimes, the 'subtle' things may mean a lot, depending on the player. How quickly the amp tracks a note, for example, or how much of a sweep in the volume knob to get it to overdrive...
Anyways, if anyone here has an amp with this kind of facility, please chime in.
 




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