question on traynor yba-1

[Bruno]

I have a traynor yba-1 bass master head and it start to humm frequently. Someone told me to change power tubes first. I opened it up and look for the schematic on the web. What I found is that the pwr tubes might be a pair of 7027a, but what's in there are one 6L6GC and one A9TX.

I bought it used so it may have been modified. I run it on a traynor yf-10 cab and it sounds great, though.
It is tube rectified(one 5AR4) and also warmed up with three 12AX7a.

My question is:
Should I switch back to the 7027s?
If not, what is this mod?
If anyone can help me out with this little mystery, it would be appreciated.
thanks

[Rob Mercure]

Bruno,

Not sure perzactly what's going on with your Traynor as I don't recognize "A9TX" as a tube designation. But in any case mismatched output tubes can produce hum as a symptom so replacing these seems to be in order. The 7027A was an upgraded, high dissipation, 6L6GC modified so that there are multiple connections to internal elements which can help with heat distribution. As best I know all current production 7027As are simply 6L6GCs with these multiple connections so there wouldn't be any advantage to purchasing a JJ 7027A over a JJ 6l6GC if the 7027A costs more - unless the way that your amp is wired takes advantage of these multiple connections (such as two different pins connected to the screen grid for instance) - and it would be an easy task for a technician to quickly rewire the socket to take either tube (but you cannot use an EL34 in a socket wired for 7027As and vice versa - sparks and blown fuses result!).

I hope this answer your questions concerning a "mystery" and a "mod" as your specific query wasn't obvious. And have fun with the amp - these guys are loud and clean creating a great platform for the pedal user.

Rob

[shredhead]

7027's sound best in this amp. Any other just won't be as ballsy and will fry.

[Gtr_tech]

7027s were just repinned 6L6s. Look at the datasheets.

Any *good* quality 6L6 will work provided the sockets are wired correctly for them. Anything sold as a 7027 these days is just that co's 6L6 guts repinned and maybe a taller bottle just for looks.

Help, Uncle Ned, My Amp Has 7027's!

[Tubebass]

Only the very early YBA-1s used 7027s. All of the later ones came with 6CA7 tubes. That said, I've used both 6L6 and EL34 in mine (a 1968 model), and both work well with the bias set appropriately. As previously noted, an amp that came with 7027s might need the tube sockets rewired.

[g1]

There should be another number on that AT9x, probably another 6L6 type or maybe 7027.
You can plug any good 6L6's in with no modifications, however they did not come with adjustable bias so you would probably want to add that. I believe some of their tube amps had adjustable screen voltage, if yours does you should also check the adjustment.

[Gtr_tech]

Adjustable *screen* voltage?? Never seen an amp that had that. What would be the point? The only way you could do that is with a tube regulated screen supply with an adjustable V ref. Again....I don't think there ever was such a thing....

[RWood]

Adjustable *screen* voltage?? Never seen an amp that had that. What would be the point? The only way you could do that is with a tube regulated screen supply with an adjustable V ref. Again....I don't think there ever was such a thing....

Guild "Quantum Bass" version of the Thunderbass did just that, using a 6GF7 tube to regulate the screens. The 8417 power tube's vulnerability was that the screens would easily short. Before abandoning that tube and switching over to 6L6s, they tried this screen regulator. It works pretty well, actually, and there is a pot to dial in the voltage. 6GF7 was a television tube, a disimilar triode.

Sorry, OP, for the detour

[Gtr_tech]

Well that is certainly a rare exception to the norm. No Traynor has ever had this.

But.....a regulated screen supply is not a bad idea. Better performance than a choke, and less prone to hum pickup. IIRC, there's a circuit in the applications section of the last RCA tube manual that uses a regulated screen supply with a 0A2 gas tube.