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 Post subject: 6K6?
PostPosted: Fri Nov 05, 2021 6:23 pm 
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Joined: Sat Apr 03, 2021 1:30 pm
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Location: Chapala, Jalisco, Mexico
I'm wondering if anyone has tried using a 6K6 tube in their Tramp. I have a couple of them lying around and I was thinking that the reduced wattage of the 6K6 could lower the overall volume level while giving me more of that cranked amp feel.

I know the 6K6 has a lower maximum plate voltage (315 vs. 350 for a 6V6) but dropping the power level a bit should compensate for that, right? I'm assuming the load and bias settings would be the same as for a 6V6.

Any advice on this is greatly appreciated.

Thanks...Jim


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 Post subject: Re: 6K6?
PostPosted: Sun Nov 07, 2021 11:31 pm 
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Joined: Sat Apr 03, 2021 1:30 pm
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Location: Chapala, Jalisco, Mexico
Well after a spending a weekend messing around with this question I'm pleased to report that the answer is yes, provided you keep the Tramp's VRM power level between 8 and 9.

The RCA datasheet for this tube is here: https://frank.pocnet.net/sheets/049/6/6K6GT.pdf

I wish I could say I started off scientifically but I didn't. I just plugged in the 6K6, kept the VRM down around 8 and started playing my Strat. It sounded great and I was able to raise the master volume a good bit and still keep overall volume at a reasonable level. My wife even told me it sounded good. The tube was running cool and there was no redplating or any other signs of trouble. I was so excited I ordered three vintage RCA blackplate 6K6s from ebay...for $25 US.

This morning I woke up thinking more scientifically and that I'd better make sure this is a good idea by doing some testing. Thanks to Uncle Doug and Blueglow Electronics for teaching me what I needed to know via YouTube.

I started out by mapping the power level settings on the front of the amp to actual plate voltages measured at pin 3. I also measured screen grid 2 voltages at pin 4. I was surprised to see that pin 3 and 4 voltages were the same throughout the range. Not sure if this is a fault with my amp or the way it's supposed to be. Anyway, here are my results:

Idle - 84v
4 - 101v
5 - 115v
6 - 129v
7 - 144v
8 - 178v
9 - 315v
10 - 420v
Max - 432v

Since the max voltage for screen grid 2 in the 6K6 datasheet is 285v, I then zeroed in on the area between 8 and 9 on the power level.

I found that halfway between 8 and 9 I got a plate voltage of 250v, well within the 6K6s operating range. With bias set at low (as for 6V6) I got the following readings.

Voltage drop- 18v
Plate Current- 24.2ma
Plate Dissipation- 6.05 watts

This checked out fine against the datasheet.

Then I increased the plate voltage to 285v, the max permitted for screen grid 2. The power level knob was sitting just below 9 on the front panel.

Voltage drop- 20.5v
Plate Current- 27.6ma
Plate Dissipation- 7.87 watts

So still well within spec. I'm sure you could run this tube harder if you don't mind shortening it's life a little. From what I've heard these old tubes were very ruggedly built.

Now to the ultimate question..."why bother with the 6K6 all?". Well, the 6K6 is about 2/3 the power of a 6V6 so if you want less overall volume it'll do the trick. I also think it's a great sounding tube. There's something to be said for getting the power section more involved when playing at low to moderate volume. It just sounds harmonically richer to me.

Cost is another factor. Vintage tubes are amazing but the price of vintage 6V6s, EL34s etc. can be prohibitive. The 6K6, a true pentode, is a forgotten tube at this point. Practically no guitar geeks know about them. Demand is low and so is the price. There are still plenty of them around because they were used in tons of radios before the 6V6 was even invented. I got my first one out of an old Hallicrafters shortwave set. You can find them for next to nothing at flea markets and hamfests. Even on ebay I was able to get 3 tested vintage blackplate RCAs for $25US. Similar 6V6s would have cost a much more.

Lastly, since we're all Tramp owners here, it's easy to do! Just dial back your VRM and enjoy! In other amps you'd have to reconfigure a bunch of stuff to run a 6K6 safely.

Salud...Jim


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