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Cathode resistor for EL84 P-P Triode wired
Cathode resistor for EL84 P-P Triode wired
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Old 22nd January 2021, 12:02 PM   #11
Diabolical Artificer is offline Diabolical Artificer  United Kingdom
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Quote:
Not just any 500R pot... a 5W WW rheostat would be needed if you don't want the pot to smoke

Quote:
Make sure the pot is capable of handling the power and wire it as a rheostat
Thought I'd pointed that out, maybe I could of been clearer.


A.
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Old 22nd January 2021, 12:50 PM   #12
FlaCharlie is offline FlaCharlie  United States
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Quote:
Originally Posted by kodabmx View Post
Not just any 500R pot... a 5W WW rheostat would be needed if you don't want the pot to smoke.

026TB32R501B1A1 CTS Electronic Components | Mouser Canada

They are so much cheaper than I expected, why not just use one per tube
If you want to save a bit more, Digi-Key has a free shipping option for the US and Canada with no minimum purchase. Shipping can really drive up the total cost, especially on small parts orders.

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Old 22nd January 2021, 03:20 PM   #13
kodabmx is offline kodabmx  Canada
Spreading triode love.
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Cathode resistor for EL84 P-P Triode wired
Currently, they offer free shipping on orders over 100$, 8$ for shipping under that. I actually prefer Digikey. I found the listing through Google though

@Diabolical Yup I missed that for sure!
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Old 22nd January 2021, 06:11 PM   #14
itsikhefez is offline itsikhefez  United States
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Cathode resistor for EL84 P-P Triode wired
Thanks all for the additional information.
To provide some context, if I were keeping the amp I would leave it as-is in UL since I think it sounds really really good.
That said, I put it up for sale and got a serious offer from an individual that wants it triode wired.
They are using horn loaded Lowther's, and mentioned that even 1V rms is enough for their preferred listening level.

BTW-- this is my favorite amp that I've built to date. The only reason for sale is because I am also working on a Baby Huey EL84 with custom OPT's from Sowter, so once that one is done I don't know if this EL84 will get much use.

Given that, I want to avoid putting any adjustment pots for ease of use for the non-DIYer.

I have Jan's Autoranger along with ARTA and will measure the output power, THD and frequency response once wired as Triode, with the existing ~8K OPT's and 270R resistors.

Changing the OPT's is not an option for the sale, too much loss there.
So the only option to mitigate some of the issues with the 8K OPT would be to increase Rk, or reduce B+ a bit?
We'll see based on the measurements.


Link to the completed build: Pictures of your Tubelab amp

Last edited by itsikhefez; 22nd January 2021 at 06:14 PM.
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Old 22nd January 2021, 06:19 PM   #15
Steve Morley is offline Steve Morley  Canada
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Saw your amp pictures on the Tubelab forum. Nice looking chassis, where did you get it? Custom or stock item?

Thanks, S.
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Old 22nd January 2021, 06:22 PM   #16
itsikhefez is offline itsikhefez  United States
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Cathode resistor for EL84 P-P Triode wired
Custom, from Landfall Systems -- Landfall Systems
Not as cheap as stock item, but cheaper than every other option I've looked at that included CNC work.
IIRC, this case was about $150 total, which included the chassis (3mm alum), brushed finish, CNC work on top/rear panels and shipping.

I've used them for the SPP, 2x TSE's, a Pass Pearl and others.
Really great product and service overall, highly recommended.
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Old 23rd January 2021, 08:27 AM   #17
Diabolical Artificer is offline Diabolical Artificer  United Kingdom
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Quote:
Given that, I want to avoid putting any adjustment pots for ease of use for the non-DIYer.
The pot is temporary, connect it, adjust till you get the right operating conditions, take it out of circuit, measure resistance, install a fixed R of the nearest value.


Andy.
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Old 25th January 2021, 02:12 PM   #18
cnau is offline cnau  Canada
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I operate my Tubelab SPP in triode with no feedback. Output transformers are 6600R Hammonds. By my calculation it operates in class A; about 3.5W to clipping on an oscilloscope. Sounds very good.

I changed nothing on the amp to do this (i.e. I kept the 270R cathode resistors).

I did need to lower the B+ voltage to 300V (to keep the EL84's in spec) which I did by putting a resistor on each leg of the power transformer secondary's before the PCB terminals.
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Old 25th January 2021, 05:07 PM   #19
itsikhefez is offline itsikhefez  United States
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Cathode resistor for EL84 P-P Triode wired
Thanks for the info cnau. Which value resistors did you add to the secondary?
Since rectification is done with SS diodes, the B+ was fairly high to begin with.
To reduce it, I added another RC filter with a chassis mount resistor.

I've rewired from UL to triode (no further changes) and the tubes look to be running a little hot. I measured about 11.6V drop across the cathode resistor on the 6P14P-EV tubes.
With B+ at 318V, this is 43mA bias and 13W plate dissipation.

I was thinking of increasing the 270R to 330R to reduce the stress.
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Old 25th January 2021, 05:16 PM   #20
kodabmx is offline kodabmx  Canada
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Cathode resistor for EL84 P-P Triode wired
It's within the spec since it's running as triode and you can add plate and screen dissipation together but by all means try it and see if you like it.

FWIW: I used to run 6P1P at 18W and they last for about a year 24/7. I dropped it from 56mA to 40mA so they last longer.
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