tale of two BFDRs

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didit
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tale of two BFDRs

Post by didit »

Hello --

A '64 Blackface Deluxe Reverb. Some long while back I found it in a pawn shop. Used and not abused. Only significant missing piece was the speaker. D120F hunt solved that. Often I plug in and play, wonder then why bother building anything. Not precisely what launches this thread. Another Blackface Deluxe Reverb here on the bench. An original '65, somewhat abused. Belongs to friend. Suffered some now reversed mods to connect both channels through reverb. It was once damp enough, so story told, necessitating replacing cabinet with a Mojotone clone. Compared playing A:B it has obvious similarities. However, also some obvious tonal differences. Doesn’t have quite the same sound or feel; similar, but just not as sweet and shimmery. Inside there are obvious and not so obvious differences. So now goal was to understand cause of differences and try to tweak the '65 closer to the '64. Top is the '64 and bottom is '65. Arrows point out noted differences: on the '65 V2 plate resistors have been replaced, PI -> power tube coupling caps and coupling cap into PI replaced, and an extra paralleled cap in the tremolo. On the '64 an extra resistor was added to the bias chain, shifting the center for the pot. We swapped tubes back and forth, hearing/feeling subtle changes but no reversal of key differences.
pairwise.jpg
Voltage measurements was the big surprise. Starting out, power transformer secondaries AC were: '65 348V; and '64 335V. And at first filter node we get 455VDC vs 415VDC. The '64 is a few volts below nominal schematic 420VDC. Bias check. Previous setting of the '65 left it at -43V, cold based on cathode current measurement. Double-check both. Swap power tubes with no significant difference. Both sets are ‘70s era ‘button’ 6V6GTA. After a round of adjustments settled on bias of -38V on ’65 & 31.5 on ’64 for ~70% dissipation. The ’65 still running higher voltage — 438V vs 416.5V.

Looking at the codes on the two power transformers, the '65 is 125P23C 1965 date code and the '64 is a 125P23B 1963 date code. Google search suggests these transformers should be the same.
65 PT.png
64 PT.png
Really. So, check the secondary DCR — ’65 is 124 ohms and the ’64 (actually ’63) is 214 ohms. Cobbled up a 50W 13K ohm dummy load to put into rectifier socket, slightly less dangerous than flying alligator leads etc. With our wall voltage 120V spot on, the ’65 was 685 VAC and the ’64 was 661VAC. A 5% difference, but one that seems to make all the difference.
dummyload.jpg
A few questions swirl. Is the difference between -23C & -23B transformers well known, but still secret to google? And onto how to tweak attempting match. It is easy to soak up 15-20V with zener/power-fet or simply dropping resistor. Will change power supply impedance, and therefore dynamics dynamics. Inclination is to start by trying out a pair of ~50 ohm resistors balanced on the secondary and before the rectifier, but open to suggestions any and all. Also open to thoughts/suggestions on whether to bother restoring to closer to original for the components in the '65.

Thanks .. Ian
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Last edited by didit on Sun Aug 04, 2019 11:07 pm, edited 1 time in total.
pdf64
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Re: tale of two BFDRs

Post by pdf64 »

How interesting, thanks for such a detailed report.
It may be that the tendency to keep upping the power output was there in Leo's day, not just a 70s CBS thing.
Yes, adding 50 ohm resistors in series with each rectifier plate seems an obvious 1st step to try and get the 'C' more in line with the 'B'.
I think that most power tube circuits tend to sound a bit sweeter with a reasonable HT.

The RGKeen article provides a rationale why CC plate resistors may help to warm the tone a fraction http://www.geofex.com/Article_Folders/c ... oncomp.htm so I suggest that changing the replacement (MF?) types back to CC may be a good 2nd step.
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didit
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Re: tale of two BFDRs

Post by didit »

Thanks sir. Will rummage the spares boxes for a few 100K CC, though skeptical on finding any. Thoughts whether 715P orange drop PI coupling caps might also need retro fitting?
68strat
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Re: tale of two BFDRs

Post by 68strat »

I know a little about your two amps. Let me read more and think about it in better to help you. I own a silverface i hacked up trying different ideas Doh! Also a friend has a blackface that i replaced electrolydic power supply caps only. Yeah i cant spell. Sprayed tube sockets with freon from radio shack, power supply caps are international ic and the amp still Sounds sweet 27 years later.

The more you solder and changing out parts on these amps the less sweet they sound!!!
My friends blackface so kicks and is quiet as a mouse. I dont want to talk about silverface except the lead dress made tons of difference on on preamp tube socket. Fat to thin moving lead 1/8 inch.

About your power transformers between two amps. Nobody i have seen thats knows more on this is a kind man that goes by old tele man. He posted his findings in ampworkshop years ago. Fenders transformers varied in voltage. If it was my blackface id quit swapping out tubes onit to test another amp. Gosh such a sweet amp. Not trying to tell you what todo
68strat
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Re: tale of two BFDRs

Post by 68strat »

No changing the 715p on pi is not gonna make alot of difference. I changed to prolly the exact same caps years ago on my silverface dr cause i noticed the speaker cone pushing and holding forward. It was dc leakage not dectecable with an analog voltmeter. You are good to go there.
Last edited by 68strat on Tue Jun 20, 2017 10:12 am, edited 1 time in total.
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didit
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Re: tale of two BFDRs

Post by didit »

Tinkering, with the '64 in particular, is risky but we've been careful and curiosity is healthy.

All 'lytics in both amps have been refreshed fairly recently, and test good. Good quality Sprague.
caps.jpg
All 16uF in the '65 as above; all 20uF in the '64. Tempted upping first node in the '65. Note the first 10K ohm resistor is discoloured from heating. That gets replaced/upgraded. Second has drifted to 12K ohm, and still deciding whether that's OK or should be swapped out too.

Decisions for the '65 are to carefully swap the "normal" plate resistors into the "vibrato" channel and put fresh resistors into the "normal". And test installing ~50 ohm before the rectifier plates. Will report back.

Best .. Ian
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Firestorm
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Re: tale of two BFDRs

Post by Firestorm »

What rectifiers are in each? Originals were RCA-branded Blackburn Mullards.
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didit
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Re: tale of two BFDRs

Post by didit »

Firestorm wrote: Mon May 22, 2017 3:06 pm What rectifiers are in each? Originals were RCA-branded Blackburn Mullards.
Should have noted this in initial post. Both are GZ34: '64 has '60s Amperex; and '65 has contemporary Mojotone, no marking for origin but rugged looking & suspect Chinese. Swapping made no discernible difference in various voltages. And with brief session of playing the Mojotone seemed to have no negative impact on overall tone/touch of the '64.

Thanks .. Ian
iwasthe1
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Re: tale of two BFDRs

Post by iwasthe1 »

I recently came across a cache of original black panel power transformers that were pulled out in the 80's to convert to send overseas. Check out the difference between a 1964 B and a 1965 D. The 64 is much smaller. Maybe that's why I always thought 64's seem more "hairy" than the other black panels.
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didit
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Re: tale of two BFDRs

Post by didit »

iwasthe1 wrote: Wed Sep 13, 2017 5:18 pm I recently came across a cache of original black panel power transformers that were pulled out in the 80's to convert to send overseas. Check out the difference between a 1964 B and a 1965 D. The 64 is much smaller. Maybe that's why I always thought 64's seem more "hairy" than the other black panels.
The dimensions of B & C are much closer. Conclusion it was mostly wrapped up in the copper inside. Poked around in a '68 that was on hand and the PT in it was larger, though didn't measure size or voltages systematically. We may have time to get back on this fall and be more thorough.

Assuming interest, might have a good home for one of the model B transformers. Let me know via PM?

Best .. Ian
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Re: tale of two BFDRs

Post by Stevem »

Fender power transformers and there voltages and related part numbers and schematics have been suspect for a long time!
Just look at the 4 super Reverb models spanning from the AA763 to AA270.
These amps are all listed schematic wise as having the part number power Trans ( 125PSD ), yet two schematics show it as having a heater center tap which it does not, they use 100 ohm balance resistors !

On top of that two of the latter models ( AB568 and AA270 ) when they switched over to 5U4 recto's ( for less voltage output and less blown speakers) show the same B+ voltage if not more then the lesser impeadance GZ34 recto of the earlier two models!

This same issue shows up with the pro Reverb schematics and model!
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tubeswell
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Re: tale of two BFDRs

Post by tubeswell »

Test for DC leakage on the coupling caps.

A lot of difference can be the speaker.
He who dies with the most tubes... wins
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didit
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Re: tale of two BFDRs

Post by didit »

Thanks. Both fine suggestions.
tubeswell wrote: Fri Sep 15, 2017 2:56 am Test for DC leakage on the coupling caps.
Did that. Might perhaps go through again with an oscilloscope vs multimeter. A better window into the signal path.
A lot of difference can be the speaker.
Yes. One of the first items. Each has decent D120F and we did semi-controlled swapping of '64 w. '65 speaker and reverse.

In the end, after putting small added resistance on HT secondary (68 ohms per side), tidying few sloppy prior-owner repairs success declared. Not 100%, but much much much closer.

Best .. Ian
mojotom
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Re: tale of two BFDRs

Post by mojotom »

didit wrote: Mon May 22, 2017 7:12 pm
Firestorm wrote: Mon May 22, 2017 3:06 pm What rectifiers are in each? Originals were RCA-branded Blackburn Mullards.
Should have noted this in initial post. Both are GZ34: '64 has '60s Amperex; and '65 has contemporary Mojotone, no marking for origin but rugged looking & suspect Chinese. Swapping made no discernible difference in various voltages. And with brief session of playing the Mojotone seemed to have no negative impact on overall tone/touch of the '64.

Thanks .. Ian
I would have advise to swap rectifier but you did. Noticed quite a different feel between a good nos one and a modern production (some Chinese are great I have to say).

I had 4 vintage SR at the same time for a while and got to play them extensively and they were all very different sound wise. One was definitely better than the other.

I would take a full voltage chart (cathodes and plates) of both amps and compare, besides the obvious main B+ voltage.
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