Single Channel Blackface Vibroverb feels too loud?

[Supertwang]

On the 40-50 watt Fender amps I like to tame them down by: 1)install a 5R4 or 5V4 rectifier. These rectifiers will drop a little voltage compared to 5AR4 and 5U4 yet provide slow start-up. 2)install 10K + 10K dropping/filtering resistors in the power supply filtering section to drop a little more voltage and improve the AC/DC filtering section. Your output tubes will thank you. 3)bias it @ 30mA instead of 40mA. Your output tubes will thank you again. 4)install the most inefficient speaker(s) you can find/afford. 5)install a proper PPIMV,….Post Phase Inverter Master Volume.
Fender had their amps (especially the Silver-Faced) tuned up to max output they could afford to warranty. With these mods completed you reduce the output from 50-ish watts into efficient speaker(s)…. TO….@ 35 watts into inefficient speaker(s). Your tubes will also last about twice as long and IMHO improves the sound. Fender amps have the tendency to BLARE. These mods cut the BLARE off at the knees.

 

[tolstoy96]

Anything look off on my voltages? This is without the trem engaged which explains the measurements on v4. Everything else looks to be in the ballpark.

tolstoy

 

[peteb]

Anything look off on my voltages?

They look all right.

Does V3 have a cathode voltage at pin 8?

 

[Wally]

It seems to me one of the cathode resistors from pin 8 or 2 of the preamp tube needs to change from 1500 ohms to 820.. for single channel? Does that ring a bell with anyone?

For a single channel V1, both pin 3 and 8 would need a 1.5K resistor for normal an 820 ohm would rebias that triode for a bit l there.
The two channel Fender reverb amps used an 820 ohm resistor for a shared cathode circuit on pins 8 of V1&V2. That 820 ohm resistor shared by those two triodes was the equivalent of using a 1.5K ohm resistor on each of two independent cathodes.

I see a big OT there. The Vibroverb used a smaller OT than did a Super Reverb or a Bassman. That will yield a bigger sounding output.
However, that quick ramping up of the volume vis-a-vis the volume pot movement indicates a problem with the taper, imho. One could use a 5751 or a 12AY7 there to change that…. Or find a different taper.
@schmee’s suggestion about the lack of a mixing situation there would be a good section to study. But…the single channel Princeton Reverb, which has virtually the same Vibrato channel preamp, does not use a resistance there?? I don’t design amps, but this area is interesting .

 

[tolstoy96]

The other issue I’m fighting is the noise introduced by the reverb circuit. Noise increases as the reverb volume is turned up. It does this with or without the tank being plugged in. Doing some chop sticking today I found pretty much all the wires going to the reverb recovery tube microphonic along with the area surrounding the 3.3meg reverb mix resister….especially the 10p silver mica cap in parallel. I have a new CTS audio 1meg pot I’m going to swap wor the Alpha, along with swapping the linear reverb pot for an audio taper. I had heard some people mention getting some bad silver micas right out the gate, so I ordered up some Vishay ceramics to replace the micas I have in there now.

Any other things I should be looking at?

Tolstoy

 

[Jsnwhite619]

I think that situation I showed was a case where the builder removed the Vibrato as well as building it single channel. The lack of channel splitting adding 6db and the lack of vibrato adding 4 more db. If that is accurate, then 6db in your situation is a lot! And may be the culprit.
I wonder if you could put a resistor higher than the 220k resistor and see what Happens? The end of those notes mentions about 38k ohms, so maybe a 250-270k resistor?
Easy to try anyway.

Not giving an opinion if this IS the reason, but if it is, then yes... 6-10dB would be huge I would think. I know that by the time you're measuring speaker output, 10dB is enormous difference. If memory is correct, I think every 10dB is equal to DOUBLE the volume?

Maybe a voltage divider would be an easy fix?

 

[tolstoy96]

Looking at Robinette’s Blackvibe which is a single channel Vibroverb, but it is the normal channel only. He is using a 270k resister sent to ground to replace the vibrato channel. I am using the stock 220k resister in mine. Would the extra 50k of resistance make much of a difference there? What if you went higher, like 330k?

Tolstoy

 

[mountainhick]

Looking at Robinette’s Blackvibe which is a single channel Vibroverb, but it is the normal channel only. He is using a 270k resister sent to ground to replace the vibrato channel. I am using the stock 220k resister in mine. Would the extra 50k of resistance make much of a difference there? What if you went higher, like 330k?

Tolstoy

Your 220K would be QUIETER than 270K to ground. A lower resistance to ground bleeds more signal to ground

 

[tolstoy96]

The 220k resister on the vibrato channel I am actually using would stay the same. The 220k resister for the normal channel which does not exist in my circuit would change. When I first fired this up I had the 220k non-existent normal channel resistor in place, but it didn’t go anywhere. The excessive output of the amp presented itself, so after some searching I saw that I should terminate that resistor to ground. So I added a jumper from that resistor to ground and the output improved, but it feels like it should still be less. It looks like Robbie changed the mixing resister on his unused channel to 270k. Are you saying that I should go the opposite direction on my unused mixing resister and lower the value?

I’m also going to swap the 820 resistor on my v1 to a 1.5k.

Tolstoy

 

[mountainhick]

The 220k resister on the vibrato channel I am actually using would stay the same. The 220k resister for the normal channel which does not exist in my circuit would change. When I first fired this up I had the 220k non-existent normal channel resistor in place, but it didn’t go anywhere.

Tolstoy

Is there or is there not another resistor to ground attached to your 220K Vibrato channel mixing resistor?

If there is not, adding a 220K might solve your entire problem. (Rob added a 270K for the tremolo channel delete, because the 220K is in series with the 50K tremolo pot to ground).

Voltage divider - Wikipedia

en.wikipedia.org

It would be much easier to help you with a schematic provided for your actual amp instead of trying to interpret word salad.

This would work:

BTW, your build looks immaculate! Nice Work!

 

[mountainhick]

Is this 220K and the wire to ground that I dotted in place?

 

[tolstoy96]

Yes, that is the 220k that would have originally fed the normal channel.

BTW, Thanks for the compliment.

Tolstoy

 

[tolstoy96]

Yes, that is the 220k that would have originally fed the normal channel.

BTW, Thanks for the compliment.

Tolstoy

 

[mountainhick]

Yes, that is it. Should I replace that wire with another 220k?

BTW, Thanks for the compliment.

Tolstoy

No, your layout is correct.

I've highlighted the voltage divider here: the pink incoming signal is reduced by half at the junction between the two 220K resistors: The signal goes through the first resistor, then half of the signal goes to ground via the blue path and half goes to the phase inverter input cap via the chartreuse green path. I would not change this. If you added another resistor to replace the black wire, the signal to the PI would INCREASE!

 

[peteb]

The DC voltages look normal.

You could measure the signal volatge.


There is a lot of uncertainty where the mixing resistor is replaced by a dropping resistor.

A good place to start is the next control grid after the dropping resistor. What does the signal voltage measure there?

 

[mountainhick]

a lot of uncertainty where the mixing resistor is replaced by a dropping resistor.

There was...

According to his photo, he has a voltage divider with good values appropriate for the deleted normal channel.

Decreasing the resistor value going to ground via the black wire, the one on the left of the "V" would bleed more signal to ground and decrease signal into the PI. That also may reduce the overdrive from the PI. Don't know if that is desirable or not.

@tolstoy96 , I know this was asked, and you've stated that the volume control acts normally, but are you dead sure the vol pot is not a linear taper? ALPHA linear would be marked as B1M

 

[peteb]

Is it too loud overall ?

Or just too loud early on the volume control.


I don’t think that it is possible for the clean output to be louder than normal.

 

[tolstoy96]

The pot is for sure marked as an A1M.

I don't know that I would say the total volume is more than it should be. But the #1 input is ridiculous right from the jump, even with my tele which doesn't have particularly hot pickups. #2 input is better, the relationship between the two seems right, but it still seems loud relative to any other blackface amp I've ever experienced, and not by just a little bit.

Tolstoy

 

[tele_savales]

The other issue I’m fighting is the noise introduced by the reverb circuit. Noise increases as the reverb volume is turned up. It does this with or without the tank being plugged in. Doing some chop sticking today I found pretty much all the wires going to the reverb recovery tube microphonic along with the area surrounding the 3.3meg reverb mix resister….especially the 10p silver mica cap in parallel. I have a new CTS audio 1meg pot I’m going to swap wor the Alpha, along with swapping the linear reverb pot for an audio taper. I had heard some people mention getting some bad silver micas right out the gate, so I ordered up some Vishay ceramics to replace the micas I have in there now.

Any other things I should be looking at?

Tolstoy

I found the .002 cap across the reverb jacks on my Vibroverb with no tremolo build to be a life saver. The blackface schematics don't have it but the silverface drawings all seem to include it. Maybe it was a lead dress issue in those 70's amps but your build looks pretty tight. Worth trying.

 

[peteb]

the #1 input is ridiculous right from the jump

It can only be the volume pot or that area of the circuit.

I assume the pot is wired correct.

It can only be the pot.

The pot is for sure marked as an A1M.

This can be verified by measuring resistances across the pot.


I don’t think there is anything more that can be done about the volume issue.