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Old 7th April 2011, 03:26 PM   #1
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Default Help identifying driver stage

I purchased what I thought was a "better" Chinese brand set of push-pull monoblocks (Melody brand, S88). They sound fine, but have a surprising amount of hum and hiss heard from the listening position. After changing tubes with no change I decided to open it up and examine the layout. It looks very well assembled however upon closer inspection I have to wonder about some of the wire routing choices, and I'm here to ask for insight into the subject.

First thing I noticed is that plate leads are zip tied to the twisted filament leads traveling to the KT88's, as well as the shielded input wire to the input tube. I re-routed the input lead assuming that was the issue. Unfortunately I hear to noticeable reduction in hiss or hum. I'm not sure if the plate leads from the output transformer need to be re-routed away from the heater leads. Can anyone help?

The input tube has it's own DC heater supply. The remaining driver and PI tubes, and KT88's have AC heaters. One KT88 and one 6SN7 per winding. I noticed that the heater leads were tightly twisted from the transformer to the KT88's, but not twisted at all from the KT88 to the 6sn7's. Also no CT and one leg grounded to the chassis (I thought they only did that in 1950's guitar amp?)
Although neatly wired I assumed this was a bad idea and rewired with twisted pairs and made a CT using 1% resistors. Surprisingly to me, I notice no reduction in hum or hiss.

Lastly, I'm trying to examine the circuit and decipher because I can not get a schematic. Here's what I've got so far:

V1 - 6SN7 SRPP, no coupling cap to V2, wired direct to grid on V2
V2 - 6SN7 Cathode coupled phase splitter (90% certain)
V3 - 6SN7 tube, but no clue what going on here. I will explain best I can:
- anodes tied together and loaded with a 51K resistor. Also an electrolytic cap is connected here on the anode pin NOT wired as a filter to the input of the 51K Rp.
- each grid is fed from a coupling cap from V2's plates
- each cathode grounded through it's own resistor.


Your help is appreciated!
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Old 7th April 2011, 04:13 PM   #2
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Looks like cathode followers.
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Old 7th April 2011, 04:38 PM   #3
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Yes I think you're correct, thank you.

Is it odd that there are no coupling caps from V1 to V2, or from V3 to the output tubes?
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Old 8th April 2011, 01:53 AM   #4
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It would be odd if there were a coupling cap between a cathode follower driver and the power tubes since that would negate most of the benefits of a cathode follower driver.

I would focus your efforts on the input stage as that will make the most difference since hum and hiss there will continue to be amplified down the chain.

How is the SRPP done? Is the heater referenced up for the upper triodes? Could this be a heater leakage issue? Maybe the DC supply is poorly done? Is it hum (50-60Hz) or buzz (100-120Hz)?

Hiss can also be a sign of excessive gain. My first tube amp was a Fisher amp with a lot of hiss and it was clipping in the 9 o'clock position. I built my own amp with just enough gain an no more and there was no background hiss at all.
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Old 8th April 2011, 02:27 PM   #5
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Thanks for your comments. The SRPP has DC heater but not biased up. I'm not sure where to add those parts but I'll take a closer look this weekend and try that. I'm hoping to avoid drilling any holes at this point. There certainly is more gain than I need but I'm not sure how to go about reducing it in any substantial way. Also, it's simply a bridge with two large uF caps after. No filter resistor between the caps. The circuit is grounded at the socket mounting screw with a heavy wire. Voltage out is 6.3Vdc.

I honestly do not know what type of hum/buzz I'm getting, I don't have the gear or knowledge to measure.

Last edited by tubesmuggler; 8th April 2011 at 02:42 PM.
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