1969 Silverface Fender Dual Showman Reverb TFL5000D

[mauiboy81]

Today I just scored the amp in the subject and was wondering if anyone has a schematic for this particular beast. I just get "error 404" at the Fender amp field guide. I believe it is from early 1969, does anyone know which circuit it would be? It is non master volume.
It needs some TLC but is a pretty sweet amp. The cabinet is beat up! The tubes are all original.
Aloha

[FYL]

The Dual Showman Reverb is a Twin Reverb in head form.

[ampdoc1]

Schematic Heaven has a DSR TFL5000 that is different from the Twin Reverb schematic here.

a'doc

[mauiboy81]

Nice! Thanks guys, I forgot about schematic heaven. Do you have any idea which schematic an early 1969 would be? I guess I'll just get in there and check it out.

[David Root]

AA768

[FYL]

Schematic Heaven has a DSR TFL5000 that is different from the Twin Reverb schematic here.

The SH schemo is for a later Red Knob version, which was built from 1987 to 1992. <del>The schemo below is period correct for an early '70s DSR w/o master volume.</del>

[mauiboy81]

Thanks guys.
That last schematic has a master volume.

[FYL]

That last schematic has a master volume.

Ooops. Should have checked it before uploading... Thank you for catching this error.

You can find schemos for the AA270, AA768 and AA769 versions on Schematic Heaven.

[mauiboy81]

No worries, I have discovered that my amp is an AA769.
Someone in the past wired a 250k pot as a variable resistor between the 68k's and before the 1k5 grid stoppers. Some kind of Master Volume? They moved the intensity pot to the back between the ext speaker jack and the vibrato pedal jack.
So is this a good way to go for a master volume or should I put it back to normal with no master?
Maybe I could add a reverb Dwell control in the hole drilled in the back? Anyone know a good way to do that?

[mauiboy81]

This is the schematic.

[FYL]

Someone in the past wired a 250k pot as a variable resistor between the 68k's and before the 1k5 grid stoppers. Some kind of Master Volume?

Probably a crossline MV, quite good in small cathode-biased amps but far from adequate in a DSR.

Maybe I could add a reverb Dwell control in the hole drilled in the back? Anyone know a good way to do that?

A dwell control isn't effective with a 12AT7-based reverb as drive has to be maxxed at all times, it'll only work as a level control.

[mauiboy81]

Thanks FYL. I will be removing the MV promptly.

If you were to add a level to the reverb what would you do?

Have you ever tried messing with the 3.3m/10pf to get a little less drastic reverb? Maybe to make the reverb control more useable across its entire range.

So how does the phase inverter circuit in the AA769 compare to the PI in the blackface? It seems to be the main difference between the two amps. Do you think it's worth blackfacing?

Thanks again!

[FYL]

.
If you were to add a level to the reverb what would you do?

I'd replace the 1M 12AT7 grid resistor with a 1MA pot.

Have you ever tried messing with the 3.3m/10pf to get a little less drastic reverb?

You may try lowering the R while boosting C. Many people like 2.2M/20pF there.

So how does the phase inverter circuit in the AA769 compare to the PI in the blackface?

Same Schmitt PI, same 12AT7, different plate values - 47K vs. 82K/100K -and a different bias point : 270R vs. 470R. I much prefer the sound with BF values, but the difference is very small at bedroom levels.

Do you think it's worth blackfacing?

SF's can sound real good - and much tighter than their BF equivalents.

I'd tidy cabling and lead dress (SF's are quite messy), remove the shunt caps in the power section (if the amp is stable as it should be with tidy lead dress), play with the tone stack caps (try 100n, 22n and 250p), modify the bias supply (level + balance instead of balance only), replace all supply R's with 3W flameproof models (lower values are a boon if you're going to play at stage levels, change the 2K2/10K for 1K5/4K7) and that'd be pretty much it. Plus of course the usual maintenance : noisy plate R's and old 'lytics should be changed.

[mauiboy81]

The amp is pretty noisy, plenty of hum and buzz.
The vibrato does not seem to do anything but tick. It does not change the signal at all but when there is no signal it ticks. The little roach blinks and the speed knob controls the speed of the blinking/ticking but the intensity does nothing.
Any ideas?

I changed the bias circuit to the blackface design but left the 68k's. Should I change them to 220k?

[Firestorm]

The amp is pretty noisy, plenty of hum and buzz.
The vibrato does not seem to do anything but tick. It does not change the signal at all but when there is no signal it ticks. The little roach blinks and the speed knob controls the speed of the blinking/ticking but the intensity does nothing.
Any ideas?

I changed the bias circuit to the blackface design but left the 68k's. Should I change them to 220k?

Do you have the footswitch? If the "vibrato" roach is lighting without it, someone has shorted the footswitch jack. You mentioned that the Intensity pot had been moved. Maybe wired incorrectly? Also the "ticking" is usually a lead dress problem (though there are some "cheats.") If the pot is still on the back, the wires are likely not dressed in a good location.

As to the 68K grid load/bias feed/grid return resistors, you can play with them pretty liberally. 68K is more polite, 220K is gnarlier (at least as gnarly as you can go with an amp that clean). Anything in between works, too.

Hum, buzz and noise would be the usual suspects: heater phase, lead dress, tubes, filter caps. When you convert to blackface bias, you lose the bias balance effect of the silverface circuit (designed so tubes did not have to be matched) so you need to check whether one or more output tubes is significantly out of whack. A few later Fenders had a neat setup: two bias controls, one set the level and one set the balance. If you were going to go to the trouble, you might as well make the outputs individually biasable.

Changing the 3.3m/10pf will alter the ratio of reverb to dry, but will also alter the gain structure of the rest of the amp (on that channel). Sure, give it a whirl, but you can also tame the reverb itself. The Dwell pot idea will change the level of the drive signal. Accutronics always said you should shake the springs as hard as you can, but not always good advice, especially with sloppy springs. Take a look at the reverb circuit in later silverfaces, especially the 135W ultralinears: instead of 2K2/25-25 on the cathode of the AT7, they use a 680R unbypassed. There's also a 560pf plate to cathode feedback cap on the AT7. I usually like the sound of that configuration better than blackface. YMMV. You can also change the tank. They do get old. Fender used long-decay tanks; you could try the medium version to clean it up, but Dick Dale would be horrified.