Bogen RP60 Distortion

[guinypig]

Hi, I'm currently working on a Bogen RP60.
- All tubes test OK
- All PSU caps have been replaced
Yet, the amp suffers heavy distortion, and I'm not certain where to look from here.
One thing I noticed is that when I turn it off, It starts to sound good while the caps are draining.
Ideas? suggestions?

 

[maxhifi]

Check all the voltages versus the schematic. Most likely a resistor is bad or possibly a bad tube socket. Also possible is a mistake when changing capacitors so double check wiring.

Oh and it could also be a bad/leaky coupling capacitor. What kind does it use? Actually that's the first thing to check.

 

[Mitch Wright]

Have the (de)coupling caps been replaced and/or are you seeing evidence of red-plating?

Have the check the resistances and voltages are in spec, as per the schematic. Naturally, you'll bleed off the voltage in the PS caps before checking resistances and take proper precautions when checking voltages.

 

[gadget73]

what kind of current are the output tubes running at? stock form this doesn't have any easy way of measuring, but breaking the connection between pin 5 and ground and installing a 10 ohm resistor at that point would be an easy way of checking. Excessive current from leaky coupling caps would show up there, same with overly low current because of weak tubes or some voltage problem. Excessive or insufficient current through the output stage will show up as nasty distortion.

I plugged the voltages into a load line calc for a 5881 (closest thing they had to a 7355) and it says with 335v on the plate in UL mode -28v on the grid is 53 ma of idle current so you should see right about a half volt across a 10 ohm resistor. Significantly more or less would point towards a problem.

if you made me throw a dart at a problem I'm inclined to agree with bad coupling caps on the output stage but if you want to diagnose it, knowing the current flow at the output stage would be a strong clue. Voltage drop across the grid resistors would be another. Good coupling caps should show very little across that 330k, bad caps will show noticeable voltage drop.
Have you tried other sources? Is it the same on both channels? Have you tried other tubes in the phase inverter spot (V14, V15). I mentioned in another thread the unconventional way that section works, I sort of wonder if a new production tube would behave correctly there. Any old production tube should be fine though.

 

[cademan]

Also check the cathode resistors.

 

[gadget73]

there aren't any in stock form. Cathode straight to ground. Fixed bias with no current adjustments, just a balance adjustment.

 

[cademan]

I have a Stromberg-Carlson amp that started doing this awhile back and found a resistor that was cracked inside. I coulda sworn it was a cathode resistor but maybe it was a power resistor. Now I'm going to hafta go and see which one it was.

 

[guinypig]

what kind of current are the output tubes running at? stock form this doesn't have any easy way of measuring, but breaking the connection between pin 5 and ground and installing a 10 ohm resistor at that point would be an easy way of checking. Excessive current from leaky coupling caps would show up there, same with overly low current because of weak tubes or some voltage problem. Excessive or insufficient current through the output stage will show up as nasty distortion.

I plugged the voltages into a load line calc for a 5881 (closest thing they had to a 7355) and it says with 335v on the plate in UL mode -28v on the grid is 53 ma of idle current so you should see right about a half volt across a 10 ohm resistor. Significantly more or less would point towards a problem.

if you made me throw a dart at a problem I'm inclined to agree with bad coupling caps on the output stage but if you want to diagnose it, knowing the current flow at the output stage would be a strong clue. Voltage drop across the grid resistors would be another. Good coupling caps should show very little across that 330k, bad caps will show noticeable voltage drop.
Have you tried other sources? Is it the same on both channels? Have you tried other tubes in the phase inverter spot (V14, V15). I mentioned in another thread the unconventional way that section works, I sort of wonder if a new production tube would behave correctly there. Any old production tube should be fine though.

Thanks for your reply.
I replaced all four coupling capacitors with new ones.
I also measured the voltage between ground and pin 5 through a 10 ohm resistor. It showed merely 1mv.

I noticed that the negative voltage of pins 1 and 6 of all four power tubes was quite different than the nominal value of -28v (~-37v). I put a pot between point H of the PSU and and the balance pots. Then, I slowly brought the voltage down closer to -28v The amp started to sound better.
Now, I am not sure that bringing down the negative voltage will really solve the problem since the B+ voltage is also more than 10 volts over the nominal value of 335.

I had not mentioned that the power tubes are 7591A which seem to have very similar specs as the 7355.

Any idea?

 

[maxhifi]

check the value of R91 and R100.
7591A is a totally different tube than the 7355, and if you are using different tubes, the output stage bias will need to be adjusted to bring the tubes out of cutoff.

 

[6l6gmike]

On my RP60 one of the PEC's for a filter switch was shorting out a channel creating a muted side. After a lot of frustration I figured it out by unhooking the input to the phase splitter tube and running it straight off a signal generator with load resistors on the outputs and o scope hooked up to them. If both sine waves on the o scope looked good then I knew that part was good. Then I worked up the chain comparing both sides until I found the area where it was muting the channel. Earlier attempts of feeding the signal generator in the the inputs and then probing down the line wasn't successful as there wasn't as much as a voltage drop in the preamp area when comparing the left and right side as a person would think.

 

[guinypig]

check the value of R91 and R100.
7591A is a totally different tube than the 7355, and if you are using different tubes, the output stage bias will need to be adjusted to bring the tubes out of cutoff.

Right. One significant difference is the Grid 1 voltage: -20v @400v for the 7591A , -34v @400v for the 7355. (Values for Class A-B)

Any help and suggestions are most welcome and appreciated.

 

[maxhifi]

Right. One significant difference is the Grid 1 voltage: -20v @400v for the 7591A , -34v @400v for the 7355. (Values for Class A-B)

Any help and suggestions are most welcome and appreciated.

1. Put a 10k linear taper pot in series with R100, connected as a rheostat (i.e. one end connects to the wiper), or just replace R100 with a 20k linear taper pot.
2. With power off and voltage bled down, measure the DC resistance across half the output transformer primary (from the plate of an output tube to the center tap), with power off.
3. Use ohm's law to compute how much DC voltage across the output transformer half will equate to 25mA idle current.
4. Connect volt meter across same output transformer primary half, and adjust newly installed pot to voltage computer in step 3.
5. Good to go - but - the new tubes have more gain than the old ones, and this may affect the stability of the amplifier. Try it with square waves and various loads to check stability.

 

[ramjet]

The schematic for the Bogen RP60 I found clearly shows that it uses 7355 output tubes. A 7591 is not a sub for a 7355 in this application without significant modification.

 

[gadget73]

sort the bias out and honestly they're not a terrible fit. Pinout is basically the same, only difference is the 7355 screen is on pin 8, a 7591 is 4 and 8. So long as nothing is on pin 4 of the socket it will work with no fuss there.

A 7591 needs less drive to make full power so the driver stage won't care. Fix the bias so the tubes conduct and it will work OK. Stability is a possible issue, more gain in the output stage means more open loop gain so more global feedback. Not too hard to check though.

but 1mv across 10 ohms means the tubes are hard off, and with about 10v too much bias that makes sense. The tubes would be operating basically in class C, conducting much less than 180 degrees. Class C works fine for RF transmitters, but its a complete no-go for audio amps.

 

[guinypig]

1. Put a 10k linear taper pot in series with R100, connected as a rheostat (i.e. one end connects to the wiper), or just replace R100 with a 20k linear taper pot.
2. With power off and voltage bled down, measure the DC resistance across half the output transformer primary (from the plate of an output tube to the center tap), with power off.
3. Use ohm's law to compute how much DC voltage across the output transformer half will equate to 25mA idle current.
4. Connect volt meter across same output transformer primary half, and adjust newly installed pot to voltage computer in step 3.
5. Good to go - but - the new tubes have more gain than the old ones, and this may affect the stability of the amplifier. Try it with square waves and various loads to check stability.

Thanks for the recipe, Maxhifi.

Adding the taper pot allowed to bring the negative voltage down to ~20v.
The amp already sounds reasonably good now, but I will make more precise adjustments using your suggestions
when I have a little more time.

Thanks for showing me the way says grateful me.

For the record, I was getting the same distorted sound using a pair of 7355 tubes which tested good.


.

 

[guinypig]

The schematic for the Bogen RP60 I found clearly shows that it uses 7355 output tubes. A 7591 is not a sub for a 7355 in this application without significant modification.

Thanks for your reply.
You are right and I was was expecting the need to make some changes.

 

[maxhifi]

Thanks for the recipe, Maxhifi.

Adding the taper pot allowed to bring the negative voltage down to ~20v.
The amp already sounds reasonably good now, but I will make more precise adjustments using your suggestions
when I have a little more time.

Thanks for showing me the way says grateful me.

For the record, I was getting the same distorted sound using a pair of 7355 tubes which tested good.


.

Make sure to adjust it properly sooner than later, because if the current is too high you can quickly cook the tubes and maybe even the transformers. If you can't get the current up to 25mA, then put another 5k in series with the new pot, to give it a bit more range.

And my pleasure!

 

[Mr Boochie]

what kind of current are the output tubes running at? stock form this doesn't have any easy way of measuring, but breaking the connection between pin 5 and ground and installing a 10 ohm resistor at that point would be an easy way of checking. Excessive current from leaky coupling caps would show up there, same with overly low current because of weak tubes or some voltage problem. Excessive or insufficient current through the output stage will show up as nasty distortion.

I plugged the voltages into a load line calc for a 5881 (closest thing they had to a 7355) and it says with 335v on the plate in UL mode -28v on the grid is 53 ma of idle current so you should see right about a half volt across a 10 ohm resistor. Significantly more or less would point towards a problem.

if you made me throw a dart at a problem I'm inclined to agree with bad coupling caps on the output stage but if you want to diagnose it, knowing the current flow at the output stage would be a strong clue. Voltage drop across the grid resistors would be another. Good coupling caps should show very little across that 330k, bad caps will show noticeable voltage drop.
Have you tried other sources? Is it the same on both channels? Have you tried other tubes in the phase inverter spot (V14, V15). I mentioned in another thread the unconventional way that section works, I sort of wonder if a new production tube would behave correctly there. Any old production tube should be fine though.

Does sound like an idle bias setting issue with the output tubes.