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Old 4th June 2006, 10:01 PM   #171
gemby is offline gemby  Croatia
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Quote:
Originally posted by gingertube
Just a hint for those building the 6V6 version with the MOSFET source followers.

I have now modified a Chinese "Music Angel" EL34 Amp (Pentode Mode) to this arrangement and have been fine tuning it over the last week (Kate's Amp).

Last night I modified the power supply to the source follower by adding a 2uF Polypropylene cap across the Mallory 40uF/500V electrolytic I used to decouple this supply - what an incredible difference, the tops became uncluttered and cleaner - the dynamic range expanded and the amp picked up rythym.

I WAS having a lot of trouble deciding between various levels of shunt feedback. Now its easy.

Rule#1 - check your power supplies FIRST.

So what about Kristy's Amp which has already been delivered?? By some fluke / good management (or whatever) I actually used a 10uF polypropylene by itself for the source follower supply decoupling in that amp.
Cheers,
Ian

Do you have maybe some schematics, photos of interior? I have same amp - still did not touch anything.
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Old 4th June 2006, 11:38 PM   #172
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gemby,
Have a look at post #21 on this thread
Pentode Push-Pull
- links to original circuit and my modified version of the Music Angel EL34. Will try to get some photos

corbato,
There are not a lot of voltages to check - these are the voltages on my amp:
Main HT Voltage: +280V
Diffamp Anodes : +170V
EL84 Cathodes: +10 to +12V

Hint: The current sources adjust the EL84 bias for matched bias currents. You can then simply select pairs of EL84s which end up with the same bias voltage to match tubes.
Example: if you had cathode voltages of 10.2 and 11.1 say on one channel and 10.4 and 10.9 on the other then rearrange the tubes to put the 10.9 and 11.1 in one channel and the 10.2 and 10.4 in the other channel.

Cheers,
Ian
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Old 5th June 2006, 05:26 AM   #173
corbato is offline corbato  India
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Thanks Ian

I am actually more keen to know the volts at Diff-pair cathodes. Planning do some experiments.
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Old 5th June 2006, 05:28 AM   #174
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corbato,
EDUCATED guess - between +1.5 and +2.0 volts with respect to 0V.
Cheers,
Ian
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Old 5th June 2006, 07:17 AM   #175
gemby is offline gemby  Croatia
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Quote:
Originally posted by gingertube
gemby,
Have a look at post #21 on this thread
Pentode Push-Pull
- links to original circuit and my modified version of the Music Angel EL34. Will try to get some photos
Tnx, i'll llok into it deeply later, but my amp i s somehow diferent, mine could not go to triode mode...but tnx anyway
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Old 5th June 2006, 11:45 AM   #176
Giaime is offline Giaime  Italy
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Hello Ian,

sorry for the partial offtopic but I haven't been able to get in contact with you in private.

I'm in the process of prototyping an EL36 PP triode amp: the input stage now is a CCS loaded ECC83 LTP, then dc coupled to a 5687 cathode follower, then ac coupling to the EL36 in fixed bias.

Now the question is: is it worth it to implement shunt feedback and partial feedback since I'm using ECC83 frontend? From this point of view, if I do this could I remove the cathode follower driver or it will degrade performance (since EL36 are not so easy like EL84). Please note that I need AT LEAST 35Vrms drive to the EL36 grids with 2Vrms input.

Thank you very much, and if you feel that this is too OT you can answer me at my email addres in my profile.
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Old 6th June 2006, 03:54 AM   #177
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Giaime,
Check out post #54 on this thread. Its the schematic for the 6V6 fixed biased variant. What you would need to do is the same thing except your tube cathode followers replace the MOSFET source followers shown (and the zener protections are then not required). You probably would need to increase the anode resistors on the 12AX7 up to say 150K or 180K and drop the current source a little to balance the DC voltages. Also you would not want to put too much shunt feedback - considering the voltage you need to swing into the output tube grids BUT perfectly do-able and will give you a little improvement in damping plus the self balancing action.

Start Point if you decide to do it:
I'd try 150K on the 12AX7 anodes to 39K from output tube anodes networks with about 10K as the cross connect (shunt feedback set) for a start and drop the CCS current to 1.5 mA (that is 750uA each side of the diff amp). Fire it up and see what happens - you want about +150V to +160V on the 12AX7 anodes
If you have plenty of volts then you might like to go 180K on the 12AX7 anodes - if you don't have enough volts then you need to drop the CCS current a little.

Cheers,
Ian
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Old 6th June 2006, 12:11 PM   #178
Giaime is offline Giaime  Italy
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Thank you very much Ian, I will modify the prototipe as soon as I get some free time.

Thank you!
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Old 6th June 2006, 12:54 PM   #179
panos29 is offline panos29  Greece
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Lightbulb PL84 instead of El84?

I know it might seem a bit irrelevant but thought could ask if it is possible to use PL84 instead of EL84. I know its a different tube but is it worth to try it soundwise? I am asking as I got about a dozen new in box PL84 telefunkens gathering dust...
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Old 7th June 2006, 12:37 AM   #180
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panos29,
PL84 could be made to work BUT there are a few issues:
1) Screen voltage limitation - you would not want to run HT of more than say +250V
2) Output Transformer Raa needs to be 4K Ohm compared with 8K Ohm for EL84
3) distortion will be a little higher

Output Power will be much the same.

Plus the 15V @ 300mA for PL84 Heaters

Cheers,
Ian
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