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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 12 Mar 2018, 22:03 
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I recently picked up this harman kardon ca-75 PA amp. After cleaning it up and replacing the capacitors I turned it on and it works, but has an awful hum which can be lessened by adjusting the balance control. After testing the 8417 tubes one of them tested bad and the other significantly worse. I'm pretty sure the hum is from the mismatched output tubes, but after looking at 8417 tube prices I want to convert the amp to a more available/cheaper tubes like KT88s. This is where I need help I'm relatively new to building tube amps. I have built a preamp and a 6550 SE power amp from schematics, but don't have much experience modifying circuits. I found a schematic for the ca-100 version of the amp and it looks the same from what I've checked against my amp. So far I know I would have to change the resistor values at the output tubes to properly bias the new tubes, but don't really know how to figure out these values. The other problem I see is the power supply not providing enough negative voltage for the KT88s. Thanks for reading, any help is appreciated.


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PostPosted: 12 Mar 2018, 22:46 
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So what's stopping you from plugging in a couple of KT88s, adjusting the bias balance, adjusting the hum pot on the filaments, and going to town? They look like close to one-to-one replacements. The pinouts are the same, filaments are the same, KT88 can handle the voltages the 8417 can, transfer characteristics are very similar. Seem like not much of a no-brainer.

Personally, I'd pick up a couple of used KT88s and give it a go.

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PostPosted: 13 Mar 2018, 02:12 
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Warning: 8417 need about -14V bias, KT88/6550 -50V. Make sure your bias range is acceptable first.
(after modding, turn the unit on with the HV wires to the doubler disconnected and insulated and measure the grids of the output tubes. Should be adj. from -40 to -60 abouts)

Cheers!

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PostPosted: 13 Mar 2018, 16:45 
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Thanks for the help. I had tried just replacing the 8417s with KT88s earlier but they started to red plate so I figured I needed to reconfigure the output circuit. It looks like I need to increase the negative bias voltage. I think I can do this by changing the current bias supply tap off the transformer and using a voltage multiplier circuit. There isn't any information I could find on the power transformer, but from the schematic I'm pretty sure that the winding outputs 20v. I drew the circuit below I think should do the trick and output around -60v. Will just this modification and the existing bias adjustment work or do you think I'll need to add a voltage divider to the bias output to get a more accurate bias?


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PostPosted: 14 Mar 2018, 01:26 
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What I do with such bias mods is use a little transformer like a Tamura 3FD-456 (Digikey MT2131-ND) on a perboard and build a bias supply with a bridge, cap, resistor and another smoothing cap. Bias controls to each tube for perfect balancing.

Go over on the filter caps... like 160V instead of 100V ones to account for line fluctuations.

It's far easier than playing around with doublers and it's $10+ a few parts :D

Cheers!

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* Ratings are for transistors - tubes have guidelines*
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