Whats going on when you don't match OT impedance exactly?

[Tonegeek]

I tried a pair of new Winged "C" 6L6 tubes in my amp that has the MC Marshall replacement tranny. Normally I use JJ EL-34's in this amp. I suspect this tranny is designed for EL-34's because it just does not sound right with anything else and i have tried 3 different sets of 6L6's with no luck although the Winged "C" were the best of the lot. The tone with the 6L6 was a bit muted on the high end compared to the EL-34. At bedroom levels up to moderate levels they sounded good with the Dumbly preamp breakup/singing/bloom thing going on. When cranked the EL-34 had smoother, more complex harmonics. The 6L6's when cranked were loud and full, but the distortion from the preamp was not coming through in a good way. It was not as sweet. I am wondering if the (old and tired) EL-34s were starting to break up which added something. The 6L6's seem like they are more transparent and just passing along what is being fed in. It got a bit harsh in the upper mids too.

I am looking at 3 areas for why the tone is not better than with the EL-34 (besides maybe I just prefer EL-34's)
1) the transformer impedance is not optimal for 6L6.
2) the 6L6 don't compress any, especially as they are new
3) the 6L6 seem to take more drive on the grid to reach the same volume level as the EL-34s (I had to crank the OD level up more. Maybe I need a bias adj. upward - actual bias is 28ma per tube with 460 volts on plates)

Mainly I want to understand what the transformer impedance has to do with any of this. BTW- I built a non HRM using 2- JJ 6L6's (with a Merc Bassman OT) and it sounded fantastic so I don't have a bias against 6L6. They just don't work in this amp and I want to know why.

I am hoping these tubes work well in my Twin conversion. I suspect they will.

[Structo]

So this is a 50w amp right?

The MC has a primary imped of 3.6K where as a Fender 50w OT will have approx 4.2K, depending which model amp OT you use.

I would think the Marshall OT might be louder since it is a lower impedance.

But, it's my understanding that 6L6 have more headroom and that EL34'w break up sooner.

Of course there is always the exception.

I have heard of guys running the Marshall PT but not so much the OT unless of course they are going to use EL34.

[Tonegeek]

So this is a 50w amp right?

The MC has a primary imped of 3.6K where as a Fender 50w OT will have approx 4.2K, depending which model amp OT you use.

I would think the Marshall OT might be louder since it is a lower impedance.

But, it's my understanding that 6L6 have more headroom and that EL34'w break up sooner.

Of course there is always the exception.

I have heard of guys running the Marshall PT but not so much the OT unless of course they are going to use EL34.

Its a 50 watter. Not so much concerned about the power loss or breakup as much as it seems the tone and "feel" is somehow affected. Mismatched impedances can cause ringing and I am wondering if something like that is going on. I have experienced this before with amps that have a 1/2 power switch where you cut the cathodes on 2 of 4 power tubes. It never sounded right to me. Put another way, an amp designed for 50 watts sounds much better than a 100 watt amp with 2 tubes turned off IMHO.

[ToneMerc]

I think if you approach the tube as a dynamic voltage device in the context of the plate load curve, a difference of 600Hz is hardly a mismatch.

Next, ask yourself this. What is the exact plate load requirement at your given plate voltage for those 6L6's? What if it's actually is 3.6k?

What if you used the 3.4K 50W Marshall OT trans with the 100V buried tap and M6 laminations and the 6L6 tone was perfect?

What if the combination just doesn't work? It sounded like you had a recipe that was working before.

[Tonegeek]

I think if you approach the tube as a dynamic voltage device in the context of the plate load curve, a difference of 600Hz is hardly a mismatch.

I assume you meant 600ohms. Agreed, not much of a mismatch

Next, ask yourself this. What is the exact plate load requirement at your given plate voltage for those 6L6's? What if it's actually is 3.6k?

Well then I would conclude (as I did years ago anyway) that math doesn't always make great tone.

What if you used the 3.4K 50W Marshall OT trans with the 100V buried tap and M6 laminations and the 6L6 tone was perfect?

I would conclude that I got lucky!

What if the combination just doesn't work? It sounded like you had a recipe that was working before.

It doesn't work and yes it was working before, but that's not really what I am after. What I am getting at here is I want to know WHY. I guess I wasn't that clear in the original post. I am not trying to fix anything (hey it sounds great with EL-34's), just trying to understand the change in one variable called output tubes. In that sense I guess I am looking for a bit of an engeneering explanation as to why 6L6's don't seem to work that well in this amp. I offered up some speculation (impedance, etc.) to get the ball rolling, but I don't really know. I bet someone here does and that's what I am looking for, the experience of this talented crew.

Thanks for your reply

[groovtubin]

I tried a pair of new Winged "C" 6L6 tubes in my amp that has the MC Marshall replacement tranny. Normally I use JJ EL-34's in this amp. I suspect this tranny is designed for EL-34's because it just does not sound right with anything else and i have tried 3 different sets of 6L6's with no luck although the Winged "C" were the best of the lot. The tone with the 6L6 was a bit muted on the high end compared to the EL-34. At bedroom levels up to moderate levels they sounded good with the Dumbly preamp breakup/singing/bloom thing going on. When cranked the EL-34 had smoother, more complex harmonics. The 6L6's when cranked were loud and full, but the distortion from the preamp was not coming through in a good way. It was not as sweet. I am wondering if the (old and tired) EL-34s were starting to break up which added something. The 6L6's seem like they are more transparent and just passing along what is being fed in. It got a bit harsh in the upper mids too.

I am looking at 3 areas for why the tone is not better than with the EL-34 (besides maybe I just prefer EL-34's)
1) the transformer impedance is not optimal for 6L6.
2) the 6L6 don't compress any, especially as they are new
3) the 6L6 seem to take more drive on the grid to reach the same volume level as the EL-34s (I had to crank the OD level up more. Maybe I need a bias adj. upward - actual bias is 28ma per tube with 460 volts on plates)

Mainly I want to understand what the transformer impedance has to do with any of this. BTW- I built a non HRM using 2- JJ 6L6's (with a Merc Bassman OT) and it sounded fantastic so I don't have a bias against 6L6. They just don't work in this amp and I want to know why.

I am hoping these tubes work well in my Twin conversion. I suspect they will.

Pin 1 and 8 need to be bused on EL-34`s, are you UNBUSSING them for 6L6`s?? They will SUCK ( 6L6`s) if pin 1 and 8 are BUSSED!

jim@Omegamps.com

[Tonegeek]

I tried a pair of new Winged "C" 6L6 tubes in my amp that has the MC Marshall replacement tranny. Normally I use JJ EL-34's in this amp. I suspect this tranny is designed for EL-34's because it just does not sound right with anything else and i have tried 3 different sets of 6L6's with no luck although the Winged "C" were the best of the lot. The tone with the 6L6 was a bit muted on the high end compared to the EL-34. At bedroom levels up to moderate levels they sounded good with the Dumbly preamp breakup/singing/bloom thing going on. When cranked the EL-34 had smoother, more complex harmonics. The 6L6's when cranked were loud and full, but the distortion from the preamp was not coming through in a good way. It was not as sweet. I am wondering if the (old and tired) EL-34s were starting to break up which added something. The 6L6's seem like they are more transparent and just passing along what is being fed in. It got a bit harsh in the upper mids too.

I am looking at 3 areas for why the tone is not better than with the EL-34 (besides maybe I just prefer EL-34's)
1) the transformer impedance is not optimal for 6L6.
2) the 6L6 don't compress any, especially as they are new
3) the 6L6 seem to take more drive on the grid to reach the same volume level as the EL-34s (I had to crank the OD level up more. Maybe I need a bias adj. upward - actual bias is 28ma per tube with 460 volts on plates)

Mainly I want to understand what the transformer impedance has to do with any of this. BTW- I built a non HRM using 2- JJ 6L6's (with a Merc Bassman OT) and it sounded fantastic so I don't have a bias against 6L6. They just don't work in this amp and I want to know why.

I am hoping these tubes work well in my Twin conversion. I suspect they will.

Pin 1 and 8 need to be bused on EL-34`s, are you UNBUSSING them for 6L6`s?? They will SUCK ( 6L6`s) if pin 1 and 8 are BUSSED!

jim@Omegamps.com

It shouldn't matter either way for 6L6 as Pin 1 does not connect to anything internally. On EL-34 it connects to the suppressor grid. that grid in a 6L6 is intenally connected to pin 8.

all the best

[odourboy]

Kevin O'Connor says a couple of interesting things regarding matching load impedance to tube which I'll sort of summarize and paraphrase:
1) With a properly constructed transformer, no special care is needed with regard to impedance selector settings. Some manufacturers realize this and suggest that the ohms switch should be used as a "tone" switch.
2) A transformer is like a window between the tubes and the speaker. If the load and source impedances are perfectly matched, you get the maximum transfer of power and bandwidth. Turn the window one way or the other, i.e. mismatch the impedance high or low, and part of the power is reflected back at the tube, resulting in reduced transfer of power and bandwidth.

Don't know if that's of any use to your but there you go.

[Max]

Perhaps the discussion in the "Steve Farris ODS #075" thread concerning the nominal "mismatch" between the 4 Ohm OT and the 8 Ohm speaker in the 70ies ODS combo amps could be of some interest, too? It started with my post on page 12 of this thread:

https://tubeamparchive.com/viewtopic.ph ... 470#128470

Have a nice weekend,

Max