The Hundred-Buck Amp Challenge - Page 44 - diyAudio
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Old 3rd July 2011, 04:01 AM   #431
bst is offline bst  United States
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George, I'm sorry to hear of your sad news. Our family has also lost several of its elders within the past six months. Even when you know the time is approaching, the passing of a loved one is still difficult.

My wife and I send condolences to you and Sherri, and wish you strength and inner peace in the days and weeks to come.
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Old 3rd July 2011, 04:07 AM   #432
TheGimp is offline TheGimp  United States
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I had to go back and re-read some posts. I miss-read October as August...Duh!

Lots of time to play then.

I'm playing with Pentode/Triode combo tubes. I started collecting some I thought should be good for low power output stages some six months ago. Turns out the Soviet tubes I wanted to play with have a direct western replacement. I thought they should be good for something in the 2-4W range.

The amp challenge proved to be the impetus to actually do something with them.

In addition to them I decided to stay with Pentode/Triode combination tubes for the preamp and driver stages.

It should be easy to figure out what I'm playing with.

Output 6F4P/6DX8/6DQ8?

Input stages 6GH8/6LX8/6BL8/6U8/6KE8...

What is with the EIA base designations 9AE and 9DC?

They are the same pin out!
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Old 3rd July 2011, 04:18 AM   #433
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Originally Posted by bst View Post
In other news, I have my pentode preamp section up and running. It's working quite well with several tubes, including the 12SJ7 and 6AU6. It looks best on the scope, though, with an old type 77 pre-octal tube. Maybe I'll wind up with an amp that's REALLY retro, and only sounds good with a pre-WWII lap-steel guitar...
Yeah, those 6AU6's seem like they could have a lot of potential, inexpensive and common. Would be nice to see an amp show up those way too expensive EF86's in all those Vox's, Dr. Z's, 18 Watt's. Sort of the little guy kicking the big guy in the shins. I guess you could tell which tube I would like to see in your amp.
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Old 3rd July 2011, 09:06 AM   #434
ixe13 is offline ixe13  Canada
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Originally Posted by tubelab.com View Post

so easy to build that "even a drummer could do it" IS a challenge.
Wooaaahahahaha! that's too funny...

You know how to tell if a stage is levelled ?

When the drummer drool from both side of the mouth.
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Old 3rd July 2011, 12:15 PM   #435
Cassiel is offline Cassiel  Libya
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Quote:
Maybe I'll wind up with an amp that's REALLY retro, and only sounds good with a pre-WWII lap-steel guitar
Valco/Supro amps with pentode input are retro and still sound great with almost any modern guitar.

Last edited by Cassiel; 3rd July 2011 at 12:22 PM.
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Old 3rd July 2011, 10:05 PM   #436
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I had a dilemma. What is it to be, a bedroom amp or a, well louder bedroom amp? The self-split worked well enough and was plenty loud either distorted or through an efficient guitar speaker. But using it with a measly little 8" I wanted a little more clean volume. I wanted to build the low power circuit for this contest but ultimately I want the higher power circuit. I did not want to do two builds so I have been delaying putting solder to part.

Well I think I came up with a way of having my cake and eating it too. Mind you to get the higher power will I have to use a bit of SS boost because I will loose a gain stage for the cathodyne PI. Now just to figure out how to do it with just one switch.
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Old 4th July 2011, 01:57 AM   #437
TheGimp is offline TheGimp  United States
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I'm still plagued by oscillations.

The amp has no GFB.

I suspect either layout (breadboard) or power supply feedback. When it happens I get the signal all the way back to the grid of the first tube.

Breaking the signal path at the output of the tone stack or any point after that stops the oscillation, but disconnecting the output of the first stage to the tone stack does not, but does reduce it.

It's really frustrating
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Old 4th July 2011, 02:15 AM   #438
PB2 is offline PB2  United States
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Have you tried moving the feed for R3 back to V1?
I take it is high freq oscillation? Do you know the frequency?
You should suspect grounding also, keep the higher current output
grounding independent of the input stage grounding.

Did you consider having your gain control in front of U2? Not that it
would help the oscillation.

I take it those are triode and pentode sections of the 6DX8s, I would
think that it would be better to keep the triode sections closer to the
output stage since there is likely some leakage from the pentode to
the triode so the lower the gain the better. I'm thinking swap U1 and
U2.
If this is the issue, you are getting positive feedback through the leakage,
you could try swapping the drive for R31 and R32 to make it closer to
negative feedback. Still I think the previous solution is better - if it
works but you could try either or both.
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Last edited by PB2; 4th July 2011 at 02:31 AM.
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Old 4th July 2011, 02:21 AM   #439
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Quote:
Originally Posted by TheGimp View Post
I'm still plagued by oscillations.

The amp has no GFB.

I suspect either layout (breadboard) or power supply feedback. When it happens I get the signal all the way back to the grid of the first tube.

Breaking the signal path at the output of the tone stack or any point after that stops the oscillation, but disconnecting the output of the first stage to the tone stack does not, but does reduce it.

It's really frustrating
Not sure if this would be helpful or not, can't see it hurting though. How about running without the first tube but with the tone stack and then shorting out each capacitor or resistor one at a time. It should not effect the circuit but then again you should not be getting feedback.

Boy I can not wait to hear your amp, looks like it has a lot going for it.
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Old 4th July 2011, 02:35 AM   #440
PB2 is offline PB2  United States
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Got the power amp working and am getting 12.5W clean with 6V6s types (7408) and no feedback, 15W with a touch of clipping. Frequency response is 35 - 60KHz -3dB points and I made no effort to use large coupling caps so not sure how much of the low end limit is the transformer. I'm using the output transformer from this gutted integrated amp but also have one chosen for the final design.
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Last edited by PB2; 4th July 2011 at 02:40 AM.
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