SilverFace Deluxe Reverb Conversion

[Corona Blue]

Howdy all-
I'm working on this SFDR (A1172 circuit) that came to me modded with a SS rect, upgraded output stage caps (2 x 220 mfd), 6L6/5881 tubes, and an old style (single tap secondary) TF110 8ohm OT . The rest of the B+ rail is stock as is the rest of the circuit, including the PT. The voltages around the circuit now more closely resemble those of say a Pro Reverb. The amp had problems (bad power tube socket mainly) which I identified and repaired and now sounds great, suprisingly loud and sweet sounding! Feels like it's making about 25-30 clean watts.

However, the stock Fender ceramic 8ohm speaker is baked... and I'm wondering if perhaps this is due to either the upgraded output power of the amp and/or an impedance mis-match of the 6L6's to the OT/speaker. I was going to suggest the guy have the stock speaker reconed with at least a 30 watt voice coil, but before I do this I thought I would toss it out to you folks and see if you had any thoughts regarding the tube/OT/speaker impedance question. Should this guy be using a 4 or an 8 ohm load? Stock was 8 on the 6V6's.

Thanks for sharing your ideas or experience.

Allen

[famp69wv@yahoo.com]

Depends on the primary Z

Allen

It really depends on the primary impedance of that OT. I'm not familiar with it so I'm afraid this is the best I can do...

If its 8K into an 8 ohm load then Yes, by all means use a 4 ohm speaker to create a reflected impedance of 4K. I actually do the inverse all the time in using 6V6s with a 4k ohm OT (Putting a 16 ohm load on the 8 ohm tap for a reflected impedance of 8K)

However, if the primary impedance of that OT is 4K into an 8 ohm load 8 ohm speaker is the ticket. I'd be sure that speaker is rated for 40+ watts though if it were mine.

[MWJB]

BF/SF Deluxe reverb primary impedances are usually in the 6.5-7K range as far as I can tell. So running 6L6/5881 into 8 ohms is less than a 100% mismatch and well within limits. Plenty of people used to do this before the beefier current production 6V6s became available.

Not familiar with the OT you mention, but to check the primary impedance you need 2 meters and a voltage source (variac, signal generator). Pull the power tubes, disconnect the speaker and apply 0.5v to the OT secondaries. Measure the voltage developing on the OT primaries, end to end (14.5v-ish?). Divide that by 0.5v (29v), then square it (841), then multiply it by speaker impedance (x8 = 6728ohms...close enough).

I'd go one better and suggest a 50W speaker.

[Corona Blue]

Thanks guys! That OT seems to be a popular replacement for the smaller Fender amps and also hi-fi stuff.

I just found some info on this OT at Triode Electronics:

"Primary 6600 ohm CT with screen taps at 43%"

I guess the TF-110 is now manufactured as a 0-4-8 ohm secondary. This unit on the amp is just a single 8ohm secondary. Don't know who makes it, but Triode claims that it's made in Chicago, IL. Triode also claims that it is rated at 20 watts at full 20-20k bandwidth, and as high as 50 watts in narrower bandwith applications like guitar amps.

I ran the amp yesterday with both 4 and 8 ohm speaker loads and it sounds good either way. OT and PT both seem to stay at reasonable temperatures too. Seems happy.

Thanks for the assurance and advice!

Allen

[famp69wv@yahoo.com]

Probably good to go...

Allen

At 6K6 pri Z you should be fine to run 6L6s. At 400 to 425 volts with that pri Z they shouldn't develop more than about 30-35 watts anyway (similar to a trainwreck running the OT at 5k or 6K6 for a pair of EL34s) so the OT size may not be to much an issue. Better pop for a higher wattage speaker though.

Hey, that way you can drop a set of JJ 6V6s in anytime you want to go down to 20 watts as well...

[bob p]

Hey, that way you can drop a set of JJ 6V6s in anytime you want to go down to 20 watts as well...

that's got me thinking about modding the bias supply so it can be switched back and forth for 6V6 and 6L6...

[Corona Blue]

I have made notes of the modifications, with voltages. If anybody wants the annotated schem, let me know.

A couple of things... I believe that this mod should only be done to those SFDR's that had the later style PT's (FMI p/n 025130) which were a bit beefier, unless you actually upgrade the PT yourself. I have played this one at length and the PT doesn't seem to get hot at all. Also, you may want to upgrade the OT with the TF-110 or something similar as I suspect the original OT would not be happy for very long with a B+ of 486V (no joke), and of course a higher wattage speaker. BTW, I'm thinking that using 6V6's would be out of the question as plate voltage is up around 475V at plate current of 35ma per tube.

The sound is still very Fendery, more like a Pro Reverb but, with 5881's or TAD small bottle 6L6's, has a very round, compressed "yowl" much like the early Boogies. Awesome classic lead tone with a TS-9 or a SD-1. Great portable club amp that stands up and will be heard by all!

Allen