Question about subbing 6V6s for 6L6s

[f13ndus]

I've got a Pignose G40V, and I want to replace the 6L6 power tubes with a pair of JJ 6V6S. I can rebias the amp for the tubes easily enough, but I'm curious about the difference in output impedance between the two types of tube. The output transformer, with the 6L6 tubes, has two speaker taps, for 4ohm and 8ohm. The speaker I'm using right now is 8ohm. When swapping in 6V6 tubes, is the impedance seen by the transformer higher or lower than with the 6L6 tubes?

In other words, will the speaker taps want to see higher or lower loads than they're currently labelled? Does the impedance seen by the transformer change enough that I should I plug the 8ohm speaker I have into the 4ohm jack? Or would it be the other way, and I should plug a 4ohm cabinet into the 8ohm jack? Or should I not worry about it?

Also, along the same lines, I've got a Gibson GA-8 that runs two 6V6 tubes in parallel. How does that affect the relationship between tubes -- OT -- speaker? I've looked a bit at the Angela Super 6V6 amp, which is very close design-wise to my GA-8, and they say the primary impedance required is half that of a single 6V6 amp. So if I wanted to pull one 6V6 from my GA-8, am I correct in thinking that, since the primary impedance at the OT would be doubled, I should then use a 16ohm speaker instead of the stock 8ohm?

This is one of the aspects of tube amp theory I'm still shaky on, so I appreciate anyone helping educate me on this!

[hasserl]

First of all, the transformer doesn't "see" an impedance, it transforms one. The tubes see the impedance from the speaker that is transformed by the OT from a low Z to a high Z.

6V6's generally work best with about twice the impedance of 6L6's, so the amp would work best now if you considered the 4 & 8 ohm taps on your GV40 to be 8 & 16 ohm taps; doubling the load. I.e. if you're running an 8 ohm speaker, switch the impedance selector to the 4 ohm tap.

[f13ndus]

First of all, the transformer doesn't "see" an impedance, it transforms one. The tubes see the impedance from the speaker that is transformed by the OT from a low Z to a high Z.

Well, that's what I meant, just didn't know how to say it thanks for clarifying that for me, hate it when I can't properly say what I'm thinking!

6V6's generally work best with about twice the impedance of 6L6's, so the amp would work best now if you considered the 4 & 8 ohm taps on your GV40 to be 8 & 16 ohm taps; doubling the load. I.e. if you're running an 8 ohm speaker, switch the impedance selector to the 4 ohm tap.

Cool, sounds like I was heading in the right direction! So the two OT taps are connected to output jacks, labelled for the 6L6 impedances (4ohm and 8ohm)... let's say I (or, more likely, someone I lent the amp to) hooked up a speaker after swapping the amp to 6V6s, and an 8ohm speaker was plugged into the 8ohm-labelled jack (which would now be 16ohm), could it cause damage to the tubes or amp, or would it just be an efficiency issue?

I suppose the easiest solution to that problem is to relabel the jacks... or not lend the amp to anyone else in the band...

[f13ndus]

Also, along the same lines, I've got a Gibson GA-8 that runs two 6V6 tubes in parallel. How does that affect the relationship between tubes -- OT -- speaker? I've looked a bit at the Angela Super 6V6 amp, which is very close design-wise to my GA-8, and they say the primary impedance required is half that of a single 6V6 amp. So if I wanted to pull one 6V6 from my GA-8, am I correct in thinking that, since the primary impedance at the OT would be doubled, I should then use a 16ohm speaker instead of the stock 8ohm?

Thinking a little bit more about the GA-8, since 6V6s like to see twice the impedance that 6L6s do, and the amp is running two 6V6s in parallel (which should half the primary impedance used), couldn't I pull both tubes and plug in 1 6L6? Since there are two 6V6 tubes, I'd wager the PT wouldn't have any problem with 1 6L6, right?

[MWJB]

Ga8 has a common cathode resistor, so if you just run one tube of either type, they will go nuclear! Remove the cathode bypass resistor & cap at 6v6 Pin 8s, remove any jumper between pin 8 of the 2x 6V6 sockets. Buy 2x 10W resistors at twice the value of the resistor you just removed (400-470ohms) and connect one of these from each pin 8 to ground, bypass each resistor with a cap of 20-25uf 50-100v rating.

Pin 8 of each power tube socket now has it's own 400-470ohm resistor & 25uf/100v cap to ground, the 2 6V6 sockets are not tied together at pin 8.

You can now use either 2x6V6 or 1x 6L6 as the mood takes you.

[f13ndus]

I might just have to do that! I'm trying to keep the amp as original as possible, but I can always swap the original cap and resistor back in... thanks for the heads up!

Since you know something about the amp, let me ask you: I did replace a burnt resistor in this amp, that was mounted across the rectifier socket... don't have the amp or schematic with me here at work so I can't check which pins... that was before I took the time to learn more about how tube amps work (was just at the "replace bad part with good part" stage), and I haven't had to go back inside the amp for a while, so I'm not even really sure what it's for...! but if you know the amp (or have a schematic) do you know what value that resistor should be? And what does it do? It's hard to trace the circuit out with that mess of PTP wiring (how great a turret board would be!) I measured it as around 240ohms, but it was in such bad shape I'm not sure how close that is to the original value...

[MWJB]

There's no value listed on the schem. It looks like the B+ dropping resistor between rectifier pin 8 & OT B+ connection. Let's say that it's got to drop 25v at 90mA-ish = 220ohms to 300ohms sounds about right (270?). Voltages have gone up some since the schem was drawn, so I'd use a 6-8W wirewound if you don't want it to burn up again. There should be a filter cap at each end of this resistor.

[f13ndus]

that sounds about right. I replaced the filter caps when I did the work, I think I put in a 250ohm 10w, I'll double check when I get home, so that should be good to go for a long time!

I'm still on the fence about modding the cathode resistor/cap to two individuals... on one hand I'd like to keep the amp as original as possible... on the other hand, it'd be nice to be able to use it with a 6L6 for a different sound... and the cabinet finish isn't original so it's not like it'll affect the value... I really should just clone the circuit in another chassis so I can mess with it as much as I want (and with a better wiring layout!) or go ahead and build the Angela Super SE amp...

Anyway, thanks for the help!

[hasserl]

So the two OT taps are connected to output jacks, labelled for the 6L6 impedances (4ohm and 8ohm)... let's say I (or, more likely, someone I lent the amp to) hooked up a speaker after swapping the amp to 6V6s, and an 8ohm speaker was plugged into the 8ohm-labelled jack (which would now be 16ohm), could it cause damage to the tubes or amp, or would it just be an efficiency issue?

No, I don't think it will damage the amp. It will work the tubes a little harder, but those JJ's 6V6S's are stout, I think they'd be fine in that situation.

[f13ndus]

Yeah, I wouldn't be considering the swap if not for having a few pairs of the JJ 6V6S already. From everything I've read they can take a heck of a lot of punishment...

I've got a well matched pair of 1954 RCA 6V6 blackplates that I pulled from something years ago, that'd I'd never use in this Pignose... I use them in the GA-8 from time to time (recorded with them a few times) but mostly keep them boxed up and wouldn't gig with them... I know they're supposed to be stout just like any NOS America tubes, but I love how they sound and won't take chances since they're pretty much unreplaceable (on my current guitar budget) if I fried them!

Think I'm going to go ahead and order the parts to swap the GA-8 to individual cathode resistors this weekend... I used to have an Epiphone Electar 10 that I modded and sold and kind of wish I hadn't, it'll be nice to be able to get that cranked class A 6L6 tone if I want it!

Thanks again for all the info, learned enough about tube theory to keep me busy for a while, I'm sure I'll be back with more questions later.