68 12,000 with 2" high impedance OT

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julkke
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Re: 68 12,000 with 2" high impedance OT

Post by julkke » Tue Nov 27, 2012 1:04 am

I'll second that. Although when I'm practicing I like to use the PPIMV and it gives great tone but sometimes you just don't wanna hear the amp, but FEEL it hitting your chest! :mrgreen:

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Tue Nov 27, 2012 6:34 pm

Well I got the last 5 Iskras and they are awesome! Thanks to Shakti and Red Star (Dusko) for the trades, really put my amp over the top in a vintage way! Put them in last night and finished up the board except for the bias resistor that goes to the trim pot. I have a 68k Iskra for that but if I run a Variac and put the B+ around 400v then I don't think the bias would be hot enough. I've got 56k and 47k Pihers but I also have a small bodied 47k Iskra that was sold to me as a small bodied 1/2 watt (looks like 1/3 or 1/4 watt?). Would that work for this position? How much voltage is present here? How many mv do 6CA7s like to see?

Inquiring minds want to know...
Last edited by emmjaydubya on Fri Dec 07, 2012 12:07 am, edited 1 time in total.

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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Thu Nov 29, 2012 11:37 am

emmjaydubya wrote:Well I got the last 5 Iskras and they are awesome! Thanks to Shakti and Red Star (Dusko) for the trades, really put my amp over the top in a vintage way! Put them in last night and finished up the board except for the bias resistor that goes to the trim pot. I have a 68k Iskra for that but if I run a Variac and put the B+ around 400v then I don't think the bias would be hot enough. I've got 56k and 47k Pihers but I also have a small bodied 47k Iskra that was sold to me as a small bodied 1/2 watt (looks like 1/3 or 1/4 watt?). Would that work for this position? How much voltage is present here? How many mA do 6CA7s like to see?

Inquiring minds want to know...
For bias resistors, I have gone 27k, 47k, 56k and settled back at 47k. I am sitting pretty cold at about 20 mv with 349 v on the plates with my variac. I know I can go higher but I don't to unduly stress my old Sylvania 6CA7s. The tone isn't that compromise, IMHO, YMMV.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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Re: 68 12,000 with 2" high impedance OT

Post by julkke » Thu Nov 29, 2012 11:56 am

rgorke wrote:
emmjaydubya wrote:Well I got the last 5 Iskras and they are awesome! Thanks to Shakti and Red Star (Dusko) for the trades, really put my amp over the top in a vintage way! Put them in last night and finished up the board except for the bias resistor that goes to the trim pot. I have a 68k Iskra for that but if I run a Variac and put the B+ around 400v then I don't think the bias would be hot enough. I've got 56k and 47k Pihers but I also have a small bodied 47k Iskra that was sold to me as a small bodied 1/2 watt (looks like 1/3 or 1/4 watt?). Would that work for this position? How much voltage is present here? How many mA do 6CA7s like to see?

Inquiring minds want to know...
For bias resistors, I have gone 27k, 47k, 56k and settled back at 47k. I am sitting pretty cold at about 20 mv with 349 v on the plates with my variac. I know I can go higher but I don't to unduly stress my old Sylvania 6CA7s. The tone isn't that compromise, IMHO, YMMV.

20mv at 349v? Sounds really low.

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Re: 68 12,000 with 2" high impedance OT

Post by Strat78 » Thu Nov 29, 2012 1:52 pm

Your not using the 1ohm resistors to ground to measure bias so we need to go over how to bias the old fasion way (better but more dangerous; separates the men from the boys! :lol: ).
Here is a good start to begin to understand this method thanks to Robert it's pretty easy but remember you are waving around your other meter probe with a live 500v.
http://forum.metroamp.com/viewtopic.php?f=28&t=23950&" onclick="window.open(this.href);return false;

This is how mine measures up: 17.5 devided by 378v plate voltage (=.046), times 2(=.092), times 16, = 1.48 between hot fuse (OT CT) and pin 3 of V5. I like to bump it up to 2.0vdc with the old GE fat bottles. The number 16 comes from the resistance between hot fuse and pin 3 of V5 when the amp is off.
Oh, start with 47k resistor, that should put you in range. Not sure I would trust one of those little 1/4w looking resistors, you mite have to cave and use a piher in that possition. :(

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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Thu Nov 29, 2012 3:10 pm

julkke wrote:
rgorke wrote:
emmjaydubya wrote:Well I got the last 5 Iskras and they are awesome! Thanks to Shakti and Red Star (Dusko) for the trades, really put my amp over the top in a vintage way! Put them in last night and finished up the board except for the bias resistor that goes to the trim pot. I have a 68k Iskra for that but if I run a Variac and put the B+ around 400v then I don't think the bias would be hot enough. I've got 56k and 47k Pihers but I also have a small bodied 47k Iskra that was sold to me as a small bodied 1/2 watt (looks like 1/3 or 1/4 watt?). Would that work for this position? How much voltage is present here? How many mA do 6CA7s like to see?

Inquiring minds want to know...
For bias resistors, I have gone 27k, 47k, 56k and settled back at 47k. I am sitting pretty cold at about 20 mv with 349 v on the plates with my variac. I know I can go higher but I don't to unduly stress my old Sylvania 6CA7s. The tone isn't that compromise, IMHO, YMMV.

20mv at 349v? Sounds really low.
Yeah, it is. I need to put a bigger resistor if I want it higher. I am babying my 40 year (or more) tubes.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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Re: 68 12,000 with 2" high impedance OT

Post by julkke » Thu Nov 29, 2012 11:53 pm

rgorke wrote:
julkke wrote:
rgorke wrote:
For bias resistors, I have gone 27k, 47k, 56k and settled back at 47k. I am sitting pretty cold at about 20 mv with 349 v on the plates with my variac. I know I can go higher but I don't to unduly stress my old Sylvania 6CA7s. The tone isn't that compromise, IMHO, YMMV.

20mv at 349v? Sounds really low.
Yeah, it is. I need to put a bigger resistor if I want it higher. I am babying my 40 year (or more) tubes.
I thought that underbiasing was also bad for tubes but I'm not sure, will have to dig around.

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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Fri Nov 30, 2012 7:44 am

What tubes at 40 years old? GE's?
12000 Metro Kit

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Re: 68 12,000 with 2" high impedance OT

Post by rgorke » Fri Nov 30, 2012 4:28 pm

vanhalen5150 wrote:What tubes at 40 years old? GE's?
Sylvanias from the 70s, ok maybe 30.

Is underbiasing bad too?
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Re: 68 12,000 with 2" high impedance OT

Post by vanhalen5150 » Sat Dec 01, 2012 4:26 am

I'm not sure how underbiasing el34/6ca7 is effected, but other tubes types like 6L6's can be biased pretty low. My old 5150 was biased at about 25ma! Apparently that's the rough factory setting.
12000 Metro Kit

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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Sat Dec 01, 2012 10:41 am

Strat78 wrote:Your not using the 1ohm resistors to ground to measure bias so we need to go over how to bias the old fasion way (better but more dangerous; separates the men from the boys! :lol: ).
Here is a good start to begin to understand this method thanks to Robert it's pretty easy but remember you are waving around your other meter probe with a live 500v.
http://forum.metroamp.com/viewtopic.php?f=28&t=23950&" onclick="window.open(this.href);return false;
Thanks, I have that thread saved. I was inspired by your posts on the matter and I thought I'd try it without the 1 ohm resistors. More dangerous and more time consuming but I'm not afraid of a little extra math, I have a calculator. :lol:
Strat78 wrote:Oh, start with 47k resistor, that should put you in range. Not sure I would trust one of those little 1/4w looking resistors, you mite have to cave and use a piher in that possition. :(
Funny! Piher it is. I'll start with the 47k and go from there. First I need a Variac for forming and then a set of 6CA7s. NOS Sylvanias would be awesome but after pricing 'em, that's not gonna happen. What tube source do you guys like for the EH? I would like to find a source that matches tubes in-house, rather than trusting the factory numbers. Ideas?

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Re: 68 12,000 with 2" high impedance OT

Post by vh junkie » Sat Dec 01, 2012 2:34 pm

Just use modern (Carbon Film?) resistor for the bias until you settle on value. No sense chopping up vintage ABs, Pihers or Iskras while you figure this out. Might just want to stay with a reliable modern resistor in this position anyway. If the resistors in the bias circuit fail, it is adios power tubes. Another thing to consider here is the UF4007 vs 1N4007 for the diode. Try both and see if you can hear the difference Dave F. said the UF was acceptable here, but not for the mains.
Most tube distributors by in bulk from the factory and match in house(tubesandmore(valvestorm source), TubeDepot, etal). I like the EH 6CA7s about 40 PC reading, lower and no higher than 45 PC. Strat78 might chime in on his fav ranges for these.
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Re: 68 12,000 with 2" high impedance OT

Post by emmjaydubya » Sat Dec 01, 2012 9:57 pm

Carbon film on my board?!? Are you mad?!?! :D That's good advice. Pretty sure I have those values, that's what I'll try. I found this thread a while back on diodes:

http://forum.metroamp.com/viewtopic.php?f=4&t=14392

Pretty interesting the differences in sound on the diodes. I put a 1 amp UF4007 in the bias position but used the slower 3 amp 1N5408s on the power board. What combos have you guys used? I'll give the regular 1N4007 a listen on the board after the amp is broken in a little. And I'll ask for the 40PC reading on a quad from TubesandMore, thanks for the heads up.

BTW, still waiting on my 3.8H choke from MM... tap, tap, tap...

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Re: 68 12,000 with 2" high impedance OT

Post by Xplorer » Tue Dec 11, 2012 8:58 pm

wonderful ! got to read this thread ...

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Re: 68 12,000 with 2" high impedance OT

Post by neikeel » Thu Dec 13, 2012 1:43 pm

vh junkie wrote: Another thing to consider here is the UF4007 vs 1N4007 for the diode. Try both and see if you can hear the difference Dave F. said the UF was acceptable here, but not for the mains.
Do you have a reference for that quote - I would like to read. I have UFs in two amps, one of which sounds great and would not want to change the other not so that I am going to fiddle with when I get chance!
Neil

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