6V6 OT Impedance

[Jana]

Disclaimer: Ciphering is not my strong point.

Fender used a 6K6 impedance for the Deluxe OT. It didn't have a 4 ohm speaker tap but just a tap paralleled with the 8 ohm tap. So, if you plugged in an 8 ohm extension speaker, the OT impedance is 3K3. This is much lower than the design recommendations but it seemed to work. Did the tubes handle this over the long run?

You may have seen my JTM 15 thread. I have a Deluxe 6K6 OT in the amp. I was playing it this morning and decided to switch it to the 16 ohm tap with an 8 ohm speaker load to simulate the 3K3 load mentioned above. Oh my, I like it much better. The bass is tighter. The amp seems to take on a different character, which I like. Granted, this is at low volume levels.

What gives? Is this a disaster waiting to happen? I've never owned a real Deluxe so I don't have any experience with the long term effects of this (on tubes and the amp).

I'm thinking of changing the PT in this amp to drop the B+ to about 360 volts. I'm also considering putting in an OT with a 4200 ohm impedance. Am I crazy? (relative to this idea, not life in general--we already know the answer to that).

[guitardude57]

You want the load resistance on a 6V6 OT to be between about 6k6 and 10k (with 7-8k being optimal), and the load resistance on a 6L6 OT to be about 2k5 - 6K6 with about 3k3-4k being optimal. So if you use (say) a 25-45W OT with a 8K Pr Z and 4R, 8R, and 16R secondary taps, you should be able to get there either way by swapping the speaker load on the secondary.

Read from a response from Tubeswell on another forum.
Using 6V6 and 6L6 interchangeably.

Hope this helps you some.

Mike

[PaisleyTube]

imo: ain't no "right" primary impedances regarding toneshaping gear like guitar-amplifiers

so, as long it ain't too crazy (>100% down or more than 200% or so up) just use what you like.

[Jana]

Mike,
Thanks for the information, but I'm familiar with changing the secondary load to change the primary load of an OT and also with the generally accepted impedances for various tubes.

I'm trying to understand how Leo got away with doing it "wrong" on the Deluxe.

Chris,
Thanks. Unless someone can convince me that I am headed for a meltdown, I will give this mismatch a try.

[matt h]

(deleted)

[tictac]

My understanding is that the Fender amps can handle a 100% mismatch without problems (i.e. 8 to 4 ohms etc...)

If I recall correctly THD amplifier's Tweed Bassman clone had a switch that would purposely change OT taps to vary the amps tonal character....

Another example would be the small Matchless combo amps with two EL84's are using a 4k OT (usually found in 2 tube PP 6L6 amps) instead of the "correct" 8k.....

TT

[Firestorm]

Within reason, the tubes don't really care what load you connect them to. If the OT is of good quality, it won't care either, especially in this case where the "mismatch" is toward the low side. What can happen is a constriction in bandwidth, so you may be losing some higher frequencies, maybe lower ones too, though IIRC the low end is controlled by the transformer's own inductance. The effects you hear (other than a change in power output if there is one) are almost exclusively a property of the transformer, so they may not be identical if you spec a new transformer with a lower Zpri. Of course, you may like that one even better.

[ToneMerc]

"Look at the loadlines..." gives you more of the full story.

I was of the impression tweed era deluxes ran an 8k, not a 6k6.

Correct, Tweed Deluxes used the Triad 108, which is 8K/8 ohm.

TM

[pdf64]

My guess is that a DR running at 420V or more on plate and screens and a load impedance of 3k3 will cause its 6V6 to overdissipate at some signal levels.
Unfortunately I'm too lazy to brush up on loadlines to demonstrate the above.

Whatever, yes, it's just as well such tubes are tough in that regard.
Pete

[pops]

Some silver faced bassmans don't handle a mismatch. Had an amp in and forgot to plug in the second speaker to get a 4 ohm load, one tube biased hot the other cold. When i plugged in the correct load it went right into spec.

[martin manning]

Some silver faced bassmans don't handle a mismatch. Had an amp in and forgot to plug in the second speaker to get a 4 ohm load, one tube biased hot the other cold. When i plugged in the correct load it went right into spec.

Something funny going on there... I would suspect an oscillation.

[Leo_Gnardo]

Some silver faced bassmans don't handle a mismatch. Had an amp in and forgot to plug in the second speaker to get a 4 ohm load, one tube biased hot the other cold. When i plugged in the correct load it went right into spec.

Something funny going on there... I would suspect an oscillation.

I concur. Sometimes having a gator clip lead or meter probe on a plate connection will set off an ultrasonic oscillation. If so you would see the "hash" on a 'scope. If you measure cathode current instead, using the 1 ohm or 10 ohm resistor method I doubt you would see this effect.

[Jana]

Thanks for all the information. I am going to try it with the 4200 ohm transformer I have.

On a side note: I have this transformer that I had in a 50 watt amp for about 10 years. A few years ago I stripped it down (because, once again, my have to make it more better tweaking turned it into a mess). Well, this evening I decided to open the transformer up to put new leads on it (the old ones were getting short). Guess what I found? The common for the secondary wasn't soldered! The wires were just sort of touching each other. Gasp! I had been running this transformer and it was a failure waiting to happen--a failure that would have given an open circuit on the secondary no matter what impedance was selected. This transformer was from a high-end boutique builder.

[renshen1957]

imo: ain't no "right" primary impedances regarding toneshaping gear like guitar-amplifiers

so, as long it ain't too crazy (>100% down or more than 200% or so up) just use what you like.

+1 on the comment

Best regards,

Steve

[renshen1957]

Thanks for all the information. I am going to try it with the 4200 ohm transformer I have.

On a side note: I have this transformer that I had in a 50 watt amp for about 10 years. A few years ago I stripped it down (because, once again, my have to make it more better tweaking turned it into a mess). Well, this evening I decided to open the transformer up to put new leads on it (the old ones were getting short). Guess what I found? The common for the secondary wasn't soldered! The wires were just sort of touching each other. Gasp! I had been running this transformer and it was a failure waiting to happen--a failure that would have given an open circuit on the secondary no matter what impedance was selected. This transformer was from a high-end boutique builder.

Hi Jana,

4200 ohm transformer was used by Jim Kelley with 4 6V6GTA with 490V on the plate at 60W. If you have 6L6, if you had enough heater current, you would have the same output, or if it is higher it would not be audibly difference.

If you substituted 6L6 in a DR, the tone might be different, but the output isn't any louder, nor is the output any higher.

"Mismatched" outputs will limit bandwidth, but that's a concern for hi fi, not MI applications.

Best regards,

Steve