I have a Ampeg v4 that was converted to run 6550s. Someone put one bias resistor on Pin 8 of one of the power tube sockets. I decided to add three more to check the bias of the other 3 tubes.
When I got the amp, it only have two 6550s - the guy was running it at half power - so I ordered two more. Thing is now the other two are biased at 42ma and the inner two at 24ma.
Is there anything I can do to get them biasing close to one another?
UPDATE: I'm thinking of adding an additional bias control so I can have each pair biased to get a total equal amount.
The bias circuit in my amp more closely resembles this schematic:
https://www.ampegv4.com/images/schematics/V4B.jpg
Can I remove R60 and C23 to make it more like the bias on this version?
https://www.ampegv4.com/images/schematics/V4.jpg
And then remove the connection to R36 and R35. Running a bias pot to each of the control grids. One bias control would go to the control grid for V5/6and the other for V7/8.
Or do I need to built another bias supply and connect each at rectifier bridge and split them from there to each set of tubes?
Missmatched 6550s - adding additional bias for each set.
Moderators: pompeiisneaks, Colossal
Missmatched 6550s - adding additional bias for each set.
Last edited by beedoola on Tue Mar 02, 2021 3:27 am, edited 2 times in total.
Re: Missmatched 6550s
Install four separate pots to individually bias each tube.
Dedicated coupling caps are needed for each tube as well.
Dedicated coupling caps are needed for each tube as well.
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Stevem
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Re: Missmatched 6550s
If I recall right the v4 has some 540 volts on pin 3 of each of the outputs so pulling .042 amps is very close to the hot biased limit of 70% dissipation.
I would change the main bias resistor after the diode to a tad smaller value and get those two hot tubes down to 39 or 40 ma and then just live with the other 2 tubes running cooler. Just make sure to have one of each tube on each side of the OT secondary winding.
I bet even set up like this that monster will still output 80 watts of loud clean power for a good amount of time which will let you save up for new tubes,which by the way since JJ makes 7027s now you can go back to the original outputs that amp and its bias circuit where made for !
I would change the main bias resistor after the diode to a tad smaller value and get those two hot tubes down to 39 or 40 ma and then just live with the other 2 tubes running cooler. Just make sure to have one of each tube on each side of the OT secondary winding.
I bet even set up like this that monster will still output 80 watts of loud clean power for a good amount of time which will let you save up for new tubes,which by the way since JJ makes 7027s now you can go back to the original outputs that amp and its bias circuit where made for !
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Missmatched 6550s - adding additional bias for each set.
I added a question/changed the title to reflect the question. Thanks for the replies so far.
I forgot to add I also already have one bias control - a resistor in series with pot - in place of R49
I forgot to add I also already have one bias control - a resistor in series with pot - in place of R49
Re: Missmatched 6550s - adding additional bias for each set.
Hi beedoola,
good practice is to go on with the conversation.
If you edit the original post many times, everytime it is needed to re-read everything from the beginning to understand what you have changed, and the whole sense of the replies is lost.
good practice is to go on with the conversation.
If you edit the original post many times, everytime it is needed to re-read everything from the beginning to understand what you have changed, and the whole sense of the replies is lost.
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Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Missmatched 6550s - adding additional bias for each set.
If your looking to have a separate bias adjustment for each tube, London amps has a nice kit that does not hurt the walet!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
-
Stevem
- Posts: 5144
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Missmatched 6550s - adding additional bias for each set.
If your looking to have a separate bias adjustment for each tube, London power has a nice kit that does not hurt the walet!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Missmatched 6550s - adding additional bias for each set.
I had a similar problem in an amp and I made a small pcb with 4 trimmers to individually bias each tube. It is a bit fiddly to dial in the values for all 4 when the main bias pot is still in place, but finally worked
got some PCBs left here in Germany
got some PCBs left here in Germany
Re: Missmatched 6550s - adding additional bias for each set.
I'm thinking of leaving the stock bias, as shown here below. Cutting the trace that goes to the Grids, making the stock bias supply V5/6.
Then making another of this bias circuit on a perf board. Tying into the diode bridge and ground, and then the output going to V7/8. A trimmer for each side - taking the place of R49. Will that work?
Then making another of this bias circuit on a perf board. Tying into the diode bridge and ground, and then the output going to V7/8. A trimmer for each side - taking the place of R49. Will that work?
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