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Vinyl Tube Pre-Amp Waveform Offset - Page 2

post #16 of 19
12/28/09 at 3:50am
Quote:
Originally Posted by dude_500 View Post
I could setup a 200V supply but it's by no means my preferred method since in the long term I'd have to get another transformer and upgrade my caps... I'm also running on an isolation transformer.
If you have to upgrade your caps in the long term, then you'd have to change them in the short term, too.

Some isolation transformers can be used as step ups from 120 to 240 with a bridge rectifier. Which one are you using?

Quote:
Originally Posted by dude_500 View Post
What part(s) do I adjust to lower the gain?
You can use a different tube. But, in doing so you are going to have to understand the circuit so you can make adjustments.


Quote:
Originally Posted by dude_500 View Post
How do you get the 10V CCS overhead? It seems to me the CCS's are taking about 40V if it drops from 155 to 115 volts. Is there an easy way to increase this overhead voltage?
B+ is the voltage before the CCS, not after. What is your B+?

At any rate, we know this is clipping, but we don't know where. So, it seems that the next step is to figure that out rather than making changes willy-nilly. So, to that end, you need to scope the grids and plates of the first two stages (the 6T4 should be fine) to see where the clipping is.
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post #17 of 19
12/28/09 at 5:12am
Thread Starter 
Quote:
Originally Posted by dsavitsk View Post
If you have to upgrade your caps in the long term, then you'd have to change them in the short term, too.

Some isolation transformers can be used as step ups from 120 to 240 with a bridge rectifier. Which one are you using?



You can use a different tube. But, in doing so you are going to have to understand the circuit so you can make adjustments.




B+ is the voltage before the CCS, not after. What is your B+?

At any rate, we know this is clipping, but we don't know where. So, it seems that the next step is to figure that out rather than making changes willy-nilly. So, to that end, you need to scope the grids and plates of the first two stages (the 6T4 should be fine) to see where the clipping is.
I'm running the full secondary in full wave rect so it's not a step up type.

B+ is 130 when it's hot, 155 cold before the CSS units, 115 hot at the actual plate but it sounds like that's not B+.

I just scoped grids and plates, the problem is first observed at the plate of the second 6GK5... does this indicate anything to try?

I found some 350V caps on the shelf so I can upgrade that and also have an extra hammond 380V CT transformer so I can setup a half wave supply on that to get 260V B+ if that is totally necessary, although I'd rather get it working with the current supply if possible.

I did run my current supply up a little higher using a variac, I ran up to 125VAC on the primaries and the problem didn't get any better, it just proportionally existed with respect to the increased B+ and output level. Same going down in voltage.
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post #18 of 19
12/28/09 at 5:05pm
Quote:
Originally Posted by dude_500 View Post
I'm running the full secondary in full wave rect so it's not a step up type.
So use a bridge rectifier and you'll double the voltage.

Quote:
Originally Posted by dude_500 View Post
B+ is 130 when it's hot, 155 cold before the CSS units, 115 hot at the actual plate but it sounds like that's not B+.
B+ when it is "cold" is a totally meaningless number. B+ is 130V which is too low.

Quote:
I just scoped grids and plates, the problem is first observed at the plate of the second 6GK5... does this indicate anything to try?
This indicates that the second stage is your problem. Things to try are increasing B+, and increasing the tube's bias. Post 11 in this thread tells you how to think about increasing the bias. The same information should tell you how to change to a lower mu tube. Perhaps use a 6T4 in the second spot for less gain.

Quote:
have an extra hammond 380V CT transformer so I can setup a half wave supply on that to get 260V B+
No, a full wave (two diodes) will get you just north of 200V which is fine. A bridge (4 diodes) will get you north of 400V which is too high. A half wave rectifier will get you bad sound. Make sure the transformer can supply the current necessary.
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post #19 of 19
12/28/09 at 6:26pm
Thread Starter 
Quote:
Originally Posted by dsavitsk View Post
So use a bridge rectifier and you'll double the voltage.



B+ when it is "cold" is a totally meaningless number. B+ is 130V which is too low.



This indicates that the second stage is your problem. Things to try are increasing B+, and increasing the tube's bias. Post 11 in this thread tells you how to think about increasing the bias. The same information should tell you how to change to a lower mu tube. Perhaps use a 6T4 in the second spot for less gain.



No, a full wave (two diodes) will get you just north of 200V which is fine. A bridge (4 diodes) will get you north of 400V which is too high. A half wave rectifier will get you bad sound. Make sure the transformer can supply the current necessary.
I'm running bridge rectifier on it, I was thinking that was the same as full wave... I see my mistake in terminology now.

Anyways, I increased B+ to 155V by pushing the transformer primary up on a variac and now it's working great on my record player. Now I just need to buy a transformer to maintain that voltage without the variac running 130 on the primary.



Thanks for all the help!
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