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Old 4th March 2012, 04:49 AM   #1701
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Quote:
Originally Posted by Salas View Post
5R4GY RCA I liked in my 6V6 line pre with least liking 5R4WGB Raytheon. Maybe you got one 5R4GY to test also? Very nice with still more midbass but retaining enough of 5R4GY's open sonic was 5AR4/GZ34. Here pictured with an RCA 5U4G which gave a rather more closed in result than 5R4GY but with richer midbass.
Salas,

I don't have a 5R4GY to test with, mind 5R4 sounds a bit dark. I did hear the good things about the 5R4GY, will source one to test. The 5U4G RCA sounds more vivid and balance in my system.

I read somewhere that you mentioned about adding a small telfon to C2 on version one that I have. Will a 0.047 be good??

Albert
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Old 4th March 2012, 06:09 AM   #1702
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by andyjevans View Post
With the Hammond anode chokes a bit more anode resistance isn't a big deal - plenty of inductance. It would be more so with a LL1660. I must say it sounded great at this operating point - very detailed.

andy
I believe in any case that the bass response will be impacted. Need to check formulae in order to confirm which one will be impacted most however it should be minimum
Benefits of filament starvation are worth it!
Thanks
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Old 4th March 2012, 08:50 AM   #1703
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Back to rectifiers!

- AZ1 mesh: first choice, the king. Any mesh is good - and expensive!!
- 80 is very nice and cheap for a preamp, 4 pin. 5Y3 is the octal version
- 5R4GY is very nice for mid power
- 5BY5G is nice, and two are great for higher power. Medium voltage drop
- Damper diodes are good for higher power, like 6DT4 etc, in a hybrid bridge

I went through a whole stack of GZ types and 5U4 types and still preferred the 5R4GY to all of them. They were cheap but are getting a bit pricey now. Downside is the voltage drop - they have about the highest internal resistance. Also take a 4uf cap but choke input with a tiny first cap like 0.1uF is best anyway for a preamp. For low voltage drop, damper diodes with a 40uF first cap.

andy
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Old 4th March 2012, 09:11 AM   #1704
Salas is offline Salas  Greece
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Quote:
Originally Posted by albertli View Post
Salas,

I don't have a 5R4GY to test with, mind 5R4 sounds a bit dark. I did hear the good things about the 5R4GY, will source one to test. The 5U4G RCA sounds more vivid and balance in my system.

I read somewhere that you mentioned about adding a small telfon to C2 on version one that I have. Will a 0.047 be good??

Albert
47n Teflon will be fine in bypassing C2.

Each config and pre will have its more synergistic rectifier, just watch their voltage sag to always reserve >20V Vin-Vout on the regs and trust your taste in your own system as a whole. Still we both didn't care much about 5R4 flat top, and 5R4GY is to Andy's liking too.
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Old 5th March 2012, 07:03 AM   #1705
mogliaa is offline mogliaa  United Kingdom
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Slightly off topic, as I'm currently testing an 01a stage with filament bias and was concerned with the high output resistance and its capability to drive cables and amp input stage capacitance effectively. I do like the sound of 01a and will also try the 26 again in this setup.

Having read Morgan Jones latest edition and found that he measured the MOSFET follower performance being slightly better than a Russian 6c45, I though this might be a good option to include to my preamp. Has anyone tried this?
Not sure if image will upload well, but will give it a try as I am travelling and don't have my PC with me.
The MOSFET has to be a low capacitance one -hard to find but with my supply of 170v I need a 200 VDS max one. Any suggestions?
Cheers,
Ale

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Old 5th March 2012, 07:17 AM   #1706
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Hi Ale,

The Diodes-Inc (formerly Zetex of Oldham) ZVN0545 is a low capacitance n-channel, and even has a p-channel complement, the ZVP0545. They are easy to get from the catalogue-distributors.

they are 450V rated, E-line packaged (like TO92), 0,7W handling, and have maximum 4pF Cre - which is very low for a FET. Mind that this is measured at 25V D-S, at low voltages it increases sharply. Keeping at least 20V across these (same as most any FET) boosts the performance.
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Old 5th March 2012, 08:28 AM   #1707
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Thanks Rod! My anode voltage is no greater than 100V so will have a Vds>50V with an HT of 170V to ensure low Coss.
Q2 can well be a BC549 what about Q1? Any good NPN with reasonable hfe and low output capacitance that has a VCe 200v max you would recommend?

Not sure if anyone tried this setup but was probably interested in any feedback on the sound of this...
Cheers,
Ale
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Old 5th March 2012, 08:34 AM   #1708
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Q1 just needs high-voltage durability & low Cout, so the ever-useful MPSA42 can do good service here. They actually have excellent performance, low capacitance & good sound, not to mention very low cost!
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Old 5th March 2012, 08:46 AM   #1709
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Ale, please don't forget that Q2 needs an emitter resistor to make a current-source. If you use at least two LEDs for Q2, it will be more stable.

eg: If you get about 3V on Q2 base, then emitter will have 2,4V.
Current is then 2,4/Re...... this allows you to set Re and the current to suit the circuit. If the emitter voltage is less than about 2V, stability is degraded.

Use more LEDs for Q1, too. You want about 6 to 10V across Q2 C-E for good performance.
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Old 5th March 2012, 10:12 AM   #1710
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Ops forgot about the Q2 resistor when drawing the diagram last night at the hotel.
Thanks for the tips re additional LEDs. Would you recommend swapping them for Zener diodes instead or not?
Thanks Rod for the help.

I take I may be the first one trying this setup for the DHT preamp and report the sonic results. I like the 01a very much but was a bit light on the bass and can't see it as a good stage valve due to its low anode current and high plate resistance.
Cheers,
Ale
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