Author Topic: E80CF and a new phase-splitter  (Read 1843 times)

Offline Ampdog

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Re: E80CF and a new phase-splitter
« Reply #15 on: January 18, 2022, 11:00:54 AM »
Apology! Not desiring to overwhelm - but I found the often encountered cathode leakage --to-heater in many ECF82s troublesome.  (Where I used them as input stage I had to resort to grid leak bias (20 meg  G1-common  resistance) and the cathode tied directly to common (or with a very low cathode resistor for NFB). (It is basically an R.F. tube with not much accent on heater-cathode insulation.)
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Online fredeb

Re: E80CF and a new phase-splitter
« Reply #16 on: January 19, 2022, 08:45:59 PM »
Apology! Not desiring to overwhelm - but I found the often encountered cathode leakage --to-heater in many ECF82s troublesome.  (Where I used them as input stage I had to resort to grid leak bias (20 meg  G1-common  resistance) and the cathode tied directly to common (or with a very low cathode resistor for NFB). (It is basically an R.F. tube with not much accent on heater-cathode insulation.)

Thanks for the warning @Ampdog . What is the way to detect such leakage ? My modded ST70 has a board that uses ECF82/6U8 instead of 7199 , I bought it like that . I suppose the change was made because of how expensive 7199 had become , if you can find them at all . What would the symptoms of above mentioned cathode to filament leakage be ?

Cheers
- Ari

...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ampdog

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Re: E80CF and a new phase-splitter
« Reply #17 on: January 20, 2022, 06:56:34 AM »
Usually hum. 

This could be quite soft, however, so also confirmed with an oscilloscope. (I have used a few other triode-pentodes without this symptom. Yes, the 7199 was never universally accepted; too many other similar but less expensive valves on the market.  (Although most have been developed for r.f. use, they work well in audio except at low signals eg. pre-amp input stages.)
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Online fredeb

Re: E80CF and a new phase-splitter
« Reply #18 on: January 20, 2022, 07:03:28 PM »
Usually hum. 

This could be quite soft, however, so also confirmed with an oscilloscope. (I have used a few other triode-pentodes without this symptom. Yes, the 7199 was never universally accepted; too many other similar but less expensive valves on the market.  (Although most have been developed for r.f. use, they work well in audio except at low signals eg. pre-amp input stages.)

Thanks @Ampdog , there was initially hum from the amp , with 2 different ECF82 that the amp came with , one a Zaerix and I can;t remember what the other was . I ordered some RCA 6U8A and 6AN8 , I haven't even tried the 6AN8 , but all seems good with the 6U8A .


...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Online fredeb

Re: E80CF and a new phase-splitter
« Reply #19 on: January 20, 2022, 10:02:56 PM »
That said .... There are quite a couple of other ST70 driver boards out there , when I have time , I could try a different front-end . I wonder if it was the power and output transformers that limited the Radford to 25W , compared to the rated 35W the ST70 is said to put out ? Is the Bailey front-end incapable of getting 35W @ 8ohm from the ST70 output stage ? Or did the Americans and British have different methods for measuring output power ? Does the Radford offer better bandwidth at similar power levels ? Radford appear to have been quite proud of their output transformers . Are they really that good ?








 
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Moonraker

Re: E80CF and a new phase-splitter
« Reply #20 on: January 24, 2022, 10:13:51 AM »
Tube amplifiers are expensive doorstops.
Record cartridges are expensive paperweights.

Offline Moonraker

Re: E80CF and a new phase-splitter
« Reply #21 on: January 24, 2022, 10:22:11 AM »

I quite like your idea of driving Electrostats directly , I never knew it was possible . I guess it could be dangerous if something went wrong . How does it work ? Does one disconnect the electrostat transformer ?



Off topic:

I don't see why not.  The electrostatics require a push-pull drive, I believe. 
Driving them basically off the anodes of the PP output stage should be possible.
With some fine tuning to make things safe and stable of course.

Imagine the original Quad2 driving electrostats, as intended.   
The amplifier OT steps down the impedance down to the level of typical electrodynamic LS. 
And the loudspeaker's transformer reverses the impedance ratio again. 
Kind of a waste... and distortion!
Tube amplifiers are expensive doorstops.
Record cartridges are expensive paperweights.

Offline V4lve Lover

Re: E80CF and a new phase-splitter
« Reply #22 on: January 25, 2022, 08:21:21 AM »
Thanks for the warning @Ampdog . What is the way to detect such leakage ? My modded ST70 has a board that uses ECF82/6U8 instead of 7199 , I bought it like that . I suppose the change was made because of how expensive 7199 had become , if you can find them at all . What would the symptoms of above mentioned cathode to filament leakage be ?

Cheers
- Ari

You can run PCF82 with a dropper resistor in place of ECF82, if you need any let me know, i have over 100, same for most of the other series tubes. You can have them for postage.


Online fredeb

Re: E80CF and a new phase-splitter
« Reply #23 on: January 25, 2022, 08:04:30 PM »
You can run PCF82 with a dropper resistor in place of ECF82, if you need any let me know, i have over 100, same for most of the other series tubes. You can have them for postage.

Good to see you active here @V4lve Lover;) Thanks for the offer , I may take you up on that later .

...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ampdog

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Re: E80CF and a new phase-splitter
« Reply #24 on: March 04, 2022, 11:50:57 AM »
I wonder if it was the power and output transformers that limited the Radford to 25W , compared to the rated 35W the ST70 is said to put out ?

I did not do arithmetic, but much of the reason would lie in Radford having cathode-biased output stages compared to the fixed bias in the ST70. (The cathode drop voltage must therefore be subtracted from the h.t. for the actual EL34 cathode-anode voltage in the Radford.) That is assuming the same power supply voltage - I cannot recall right now.. . .
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)