Tube Amps / Music Electronics
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|3/15/2002 7:19 PM|
||need more headroom|
Just built a 5E3 and it's GREAT... I would like a bit more headroom however.
What's the best way to achieve this ?
I have 310 v DC on the plates of the finals.
I have a choke installed in place of the 5k resistor.
The power xformer is from Doug Hoffman.
The output xformer is a New Sensor for blackface deluxe.
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|3/16/2002 5:45 PM|
|Bruce /Mission Amps
310 vdc on the plates... is that as measured to ground or from the plates to the cathodes?
Makes a difference.
With lower B+ I'd expect your amp to be very slightly overbiased and running a little bit cold if you are using a 250-270 ohm cathode resistor.
A bit more headromm can be achieved at a tonal cost, that being the "tweed Deluxe tone thing".
One of which is reducing some of the bass response and limiting the signal going into the driver triode a bit.
Give up some more voltage data and I can help out a little more.
|3/17/2002 5:32 PM|
Bruce: Thanks for your response. I had measured the voltage across the first electrolytic... the one the centertap of the OT and the B+ attach to (310 v DC.) Just double checked... plate to ground...same reading.
The voltage drop across the 250 ohm biasing resistor for the finals is 19.52 v DC.
I have 78 mills bias (19.52/250).. Right?
The tubes don't look hot and actually...the customer (what does HE know !?!?!? ) LOVES the sound of this amp... He's happy as a clam.
ps: I originally obtained blackface deluxe PT but decided that would be too high voltage... even with zenner to drop it 50v ... Now I'm wondering... I was expecting 350v DC at the plates...
|3/17/2002 6:04 PM|
|Bruce /Mission Amps
My Soulkicker amp runs a similar PT, as does the 5E3 project amp, and most guys are reporting back the B+ running around 350vdc to 370vdc.
You should try using a SOVTEK 5Y3 now... it is much stiffer then a NOS 5Y3 and you should see a nice jump in B+. If that is not enough, try a SOVTEK 5AR4.
If when using either of the stiffer recitifier tubes the B+ gets up there higher, like our 5E3 project amp, consider using a 270 ohm biasing resistor instead of the 250 ohm one.
Don't be afraid of the B+ going as high as 380vdc (as measured to ground, not to the 6V6 cathodes). Bruce
|3/17/2002 7:18 PM|
Good Man Bruce !
I wish I'd talked to you about this before I bought the more expensive PT. O well....
I just put a Sovtek 5Y3 in per your suggestion and am now seeing 342 v DC on the plates.... much better. Am also seeing 21.79 v DC voltage drop across the final bias resistor giving 87 mil bias. Your suggeston of a 270 ohm bias resistor would give 80 mils.
What do you think re: 87 mills bias ? Too high ?
It looks like the JAN Phillips 5y3 (prior rectifier) would be an excellent fix for high plate voltage with the larger PT. (I bet it would have worked out perfectly with the blackface PT.)
Thanks VERY much for your speedy help on this situation...
ps: Happy St. Pat's
|3/18/2002 6:53 PM|
Glad to hear you've got some volts in there now.
With the choke, you already have more power output and headroom than anybody at the same B+ with the stock circuit.
The 5.6K resistor does a couple things your choke can't in terms of operating point and sag, both of which contribute to the nice compression these amps have.
Leo Fender had a different agenda, but in my view the 5E3 is all about sacrificing a few watts for a better sound.
Also, about the cathode current, for tubes I have here Ip = .96 * Ic, unless they are EH tubes in which case Ip = .92 * Ic. I like to stay around 12 watts, but other people here go over 14, apparently without any problem. In other words, 87ma seems fine.
|3/19/2002 5:14 AM|
"The 5.6K resistor does a couple things your choke can't in terms of operating point and sag, both of which contribute to the nice compression these amps have."
I heard the choke was an IMPROVEMENT ?? Am I wrong ? Should I go back to the dropping resistor ?
I'm running the EH tubes... 87 mills @ 342 v = almost 15 watts per tube ! That's OK ?
Ip = .92 * Ic. Explain ?
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