Selmer Thunderbird 50 MKII

[cresur66]

Hello!

I am the proud owner of a beautiful Selmer Thunderbird 50 MKII. I love the amp dearly, but I have a reverb question. I don't know how likely it is that anyone reading this owns or has ever worked on one of these, but I feel like the reverb is a little lackluster. It has a decent amount of verb coming through, but it is kind of thin and seems like it could be beefier. I've had the amp looked over, & the reverb transformer was replaced (fender-style). All tubes are good. I've tried a few different tanks of differing types with no dramatic improvements. Comparing the schematic to a Fender amp w/ reverb, I notice that the Selmer circuit has no cathode bypass caps on the driver or recovery circuitry, while the Fender has 25/25s on both cathodes. Would it be worth adding cathode bypass caps to see what happens, or is that a bad idea? Any other thoughts on things to try to see about improving the reverb, or is just inherently lackluster on these amps?

The Selmer reverb tube is a weird one, 6BR8 pentode/triode. Just curious if anyone out there has experience with these amps & their reverb. It's pretty hard to say what the Reverb sounded like when the amp was new, especially with an amp this obscure, but I'm curious.

Thanks for any input anyone might have!

[pdf64]

... Would it be worth adding cathode bypass caps to see what happens ...

As you're happy the amp is working as it's supposed to, yes, it's worth trying either or both of the above.
Also try bypassing V4 pin3 cathode.
Be aware that increasing gain can erode stability, oscillation may result, grid stoppers may be necessary to maintain stability.

[AlNewman]

Just looking at the datasheet, the 6BR8 pentode side has a 400k plate resistance, while the 12AT7 has a 10k plate resistance.  Perhaps the fender reverb transformer isn't the best fit?

[pdf64]

Just looking at the datasheet, the 6BR8 pentode side has a 400k plate resistance, while the 12AT7 has a 10k plate resistance.  Perhaps the fender reverb transformer isn't the best fit?

Maybe, but that's not really a valid comparison.
The appropriate loadline for the 6BR8 should be ascertained.

[cresur66]

Well, I can say that the original tank (which I still have, & I believe still works) I measured as having ~200ohm resistance at input and ~170 ohm resistance at output. I initially tried replacing it with a new tank with similar spec, which sounded about the same. I tried 4FB & 4EB type tanks with similar results. The original transformer I measured (in circuit) as 690ohm primary & 105ohm secondary, though I haven't re-measured it since it's been removed from the circuit. The 4AB tank I tried with the original reverb transformer didn't work at all.

Part of the problem for me was not really knowing what the original transformer was, or what to try replacing it with. The tech I took it to said it was a Fender-style reverb & he put in a (used) Fender-style transformer/tank set. I am not really sure of the best method to calculate what the primary resistance/impedance should be on the Reverb transformer.
 

[pdf64]

... I am not really sure of the best method to calculate what the primary resistance/impedance should be on the Reverb transformer.

Turn all controls down and report the V DC at lugs 6,7,8 of V2.
Apparently the 6BR8 is a 6U8 / ECF82 with a different base pin arrangement.
Whatever it seems an unusual choice for the application, especially for a UK made amp.
ECL82 or 86 seem more appropriate.
http://frank.yueksel.org/sheets/093/6/6BR8.pdf
http://frank.yueksel.org/sheets/129/6/6U8.pdf
http://www.tubebooks.org/tubedata/HB-3/Receiving_Tubes_Part_2/6U8-A.PDF

[cresur66]

Pin 6=330
Pin 7=232
Pin 8=3.6

[pdf64]

That pentode has a 2.8 W dissipation limit, but it seems to be idling at nearly 4W.
Are the 220R cathode resistor and 27k screen grid resistor measuring ok?

[AlNewman]

Well, I can say that the original tank (which I still have, & I believe still works) I measured as having ~200ohm resistance at input and ~170 ohm resistance at output. I initially tried replacing it with a new tank with similar spec, which sounded about the same. I tried 4FB & 4EB type tanks with similar results. The original transformer I measured (in circuit) as 690ohm primary & 105ohm secondary, though I haven't re-measured it since it's been removed from the circuit. The 4AB tank I tried with the original reverb transformer didn't work at all.

Part of the problem for me was not really knowing what the original transformer was, or what to try replacing it with. The tech I took it to said it was a Fender-style reverb & he put in a (used) Fender-style transformer/tank set. I am not really sure of the best method to calculate what the primary resistance/impedance should be on the Reverb transformer.

I'm not sure I understand what you're trying to say...
The reverb worked, but you weren't happy with it, so you tried different tanks and it didn't seem to help.
Then you took it to a tech, and he changed the transformer and tank, and it works, but you're not happy with it?

Did you ever try a new 6BR8?

If you're getting readings across the original transformer and tank windings which aren't short or open, that could be a good indication there was nothing wrong with them.  Now, with a fender transformer and tank, it might be better to install a 12AT7 and copy their homework with the gain and impedance.

Uncle Doug has a video on how to measure winding ratios across a transformer.  He uses a variac, but if you're resourceful and comfortable with electricity you can find other AC sources.

[pdf64]

It seems unclear whether @cresur66 is going to make a return visit   

[cresur66]

Hey guys, sorry for the radio silence, just been busy.

I did try a few different 6br8 tubes in there, didn’t really change anything. All the resistors in the reverb circuit measure within spec.

Even if I did put in a 12at7, wouldn’t i need to add another tube for the recovery part of the circuit? I’m no fender expert, but I think when looking at a deluxe reverb schematic it used the entire 12at7 for just the reverb driver stage?

I know I’m probably over thinking all this, I should be happy the reverb sounds decent, I’m mostly just wondering what the reverb on this amp sounded like when it was new. I’ve heard that the reverb on a lot of British amps just isn’t in the same league as fenders. The 6BR8 is a weird choice… I think on earlier thunderbirds they used that tube in even a different part of the circuit.

Thanks for all your input! I have watched the uncle Doug transformer videos, but should probably revisit them. Good info in there!

[AlNewman]

Hey guys, sorry for the radio silence, just been busy.

I did try a few different 6br8 tubes in there, didn’t really change anything. All the resistors in the reverb circuit measure within spec.

Even if I did put in a 12at7, wouldn’t i need to add another tube for the recovery part of the circuit? I’m no fender expert, but I think when looking at a deluxe reverb schematic it used the entire 12at7 for just the reverb driver stage?

I know I’m probably over thinking all this, I should be happy the reverb sounds decent, I’m mostly just wondering what the reverb on this amp sounded like when it was new. I’ve heard that the reverb on a lot of British amps just isn’t in the same league as fenders. The 6BR8 is a weird choice… I think on earlier thunderbirds they used that tube in even a different part of the circuit.

Thanks for all your input! I have watched the uncle Doug transformer videos, but should probably revisit them. Good info in there!

Yes, you're right, Fender specs would be a parallel 12AT7 with a 1/2 12 AX7 as recovery.  If your original parts work, which it sounds like they might, then with in-spec parts, it should perform as intended.  Maybe that isn't what you're looking for. 

There's lots of tubes, perhaps a 12DW7 may work for you with your current transformer and tank. 

You'd have to figure out the load lines and maybe smarter people than me could help with that.  I think Tubenit has a lot of experience with 1 tube reverbs.

[cresur66]

I tried the Uncle Doug video method to try to figure out the specs of this original reverb transformer & I'm wondering if it might actually be bad after all? Just using a 12V power supply connected to the secondary, I was getting 0V on the primary side. No continuity b/w windings so I don't think it's an internal short. I tried a few different DMMs just to make sure one wasn't giving me false readings.

Any other thoughts on ways I can test this Transformer? Just in case it means anything to anyone, the transformer does have "T215" printed on it.

[AlNewman]

Is your power source AC or DC?  DC voltage won't pass across the transformer. 

[cresur66]

Oh, of course, it was a DC power supply. Boy I feel like a bonehead now, haha. To be continued.

[SEL49]

Oh, of course, it was a DC power supply. Boy I feel like a bonehead now, haha. To be continued.

Check the resistance of the secondary winding. You may have burned it open with your 12vdc torture test. A good Fender reverb transformer will read about 1Ω on the secondary. High resistance or open circuit is bad.

[cresur66]

Did a double-check & Primary (690ohm) & Secondary (105ohm) resistance readings are same as always.

[SEL49]

I've had the amp looked over, & the reverb transformer was replaced (fender-style).

Did a double-check & Primary (690ohm) & Secondary (105ohm) resistance readings are same as always.

Those resistance readings are not what you should get from a typical Fender reverb transformer.

     https://www.tubesandmore.com/sites/default/files/associated_files/p-tgr-001.pdf

[cresur66]

Right, I don't believe this was a Fender-type transformer that was originally in the amp. The tank that was in it when I got it (that I believe was original) was closest to a 4FB-type tank, which I measured as having ~200ohm resistance at input and ~170 ohm resistance at output. I tried a new 4FB that sounded about the same.

[cresur66]

It's been awhile, but I finally got around to hooking up a variac to the reverb transformer a-la Uncle Doug & got some readings.

- Winding Ratio: 2.26
- Impedance Ratio: 5.11

This is pretty far removed from a Fender reverb transformer, of course. With the plate resistance of the pentode side of the 6BR8 reverb tube being 400k, I calculate the impedance that the secondary of the transformer should be looking for would be about 78 or 80k. I don't know of any reverb tanks that have a primary rated close to that? Also, the 690ohms of the primary of this transformer certainly isn't close to 400k. The 25kohm primary of a Fender type transformer is a closer match, I guess...

Anyway, it's an unusual circuit for sure and I'm still pretty confused. Maybe the reverb by design was always just kind of tame?

[pdf64]

- Winding Ratio: 2.26
- Impedance Ratio: 5.11

This is pretty far removed from a Fender reverb transformer, of course. With the plate resistance of the pentode side of the 6BR8 reverb tube being 400k ...

Anode resistance doesn't apply, a loadline suitable loadline needs plotting.

[DummyLoad]

Bypass V4 pin 8, tank driver w/ .5uF-4.7uF - experiment to taste, typically you don't want <200Hz to tank. The recovery triode is where I'd start - V4 pin 3. Try a 2.2-4.7uF there. IMO the dry/wet mixer amp (V2) is not where you want to boost gain.


--Pete