Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: FranciscoPerez on February 09, 2012, 01:27:44 pm
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Hi!
After reading such good comments about VVR in 18watt amps I decided to add this to mine, and scale the PA and PI.
First of all, without adding the VRR circuit I separated the power rail in two and measured voltages to try to get as
close as possible to the voltages present before the changes were made.
Before:
(http://i41.tinypic.com/vcs5dj.jpg)
After:
(http://i44.tinypic.com/9gv8dv.jpg)
I'd like to ask you for help in some things I find odd.
As you can see, the voltages are something different. Should I worry about it and adjust power rail resistors to match
voltages?
First stage ecc83 plate voltages are a little bit lower. Adding the VVR circuit will make preamp voltages rise a little higher
than that when VVR pot set at max position, right?
PI tube plates are higher than before, would it be a problem?
I'd like to preserve the original tone and touch response of the amp as much as possible.
On the other hand, are bleeder resistors after the reservoir capacitor really needed? I thought bleeder resistor is used to
help discharge the reservoir capacitor when amp is switched off, so it only makes sense in the first power cap. Am I right?
Seen all around people using 220k, would it make a difference to use 270k instead?
And finally, what would the extra filtering (32uf instead of 16uf like before) in the PI preamp tube produce in terms of tonal
or touch response in the amp?
I'd be glad to hear your advice on this!
Thank you very much!
Best regards!!!
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I would wait until you install the VVR to worry about adjusting the dropping resistors. My experience has been that the VVR circuit itself will cause a slight decrease in overall B+, so it doesn't make sense to adjust the rail until you have the VVR installed and know what you have to work with.
270K bleed resistors are fine.
You need the bleed resistor on that preamp rail because those caps was discharge across the diode.
The 18W schematics I'm familiar with have 33u/33u/16u filter caps, so I think what you have drawn will have the same amount of filtering for V1.
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In case you have not seen this already, there is quite a bit of VVR information right here:
http://www.el34world.com/Forum/index.php?topic=6899.0 (http://www.el34world.com/Forum/index.php?topic=6899.0)
With respect, Tubenit
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Thanks Boots Deville.
About filtering, as you say there is the same amount of filtering for V1, but not for the PI tube (V3).
Originally, the last 16uF capacitor feeds all preamp tubes (V1,V2 and V3). After splitting the power rail in two, however, V3 is fed by a 32uF cap. I wondered if that'd make a difference in the touch response of the amp, or tone. :dontknow:
Thanks for the tip about bleeder resistors, I'll add another one in the cap feeding el84s pin 9 and wait until I install the VVR to adjust dropping resistors.
Thanks for the link tubenit!! great info in there ;)
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OK, I see, you're referring to the PI tube. If you're concerned about the a change in tone, why not leave C2 as 16uF? Why the change to 32uF?
I doubt you'll notice a difference. I know *I* wouldn't be able to notice a difference.
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Nice to hear that! I only had one 32+32uf cap around, and wanted to make sure that using it wont make a noticeable difference. Thanks for that!!
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Hi!
Finished adding VVR to the amp.
Voltages with VVR pot turned all the way up are really close to what I previously had before splitting the B+ rail and adding the VVR circuitry. Now B+ in power amp tubes and PI decrease as I turn the VVR pot down as expected, so at a first glance things seem to be working good.
However, now the amp has a strange ugly sound when overdriven, specially noticeable when vvr por is turned low. It's like a background harsh distortioned sound is added to the normal sound of the amp, quite unpleasant and not very musical.
Read a little about the subject, and found the Paul Ruby's Zener mod. Just in case blocking distortion was my problem, i performed this mod, but unfortunately withou success, this harsh sound is still here.
What do you guys think that could be causing this bad distortion sound??
I should say that after performing the VVR installation I swaped tubes by mistake, and put the ez81 in place of an el84 and one of the el84s in the ez81 tube socket. Then powered up the amp for a few seconds to notice there was no sound at all, turned off the amp, and realised that i swaped those tubes.
Do you think I could have damaged the el84, and that's the origin of this bad overdriven sound? Don't have additional el84's tubes available right now, so I couldn't change them to check.
Any suggestion would be strongly appreciated!
Thank you very much you all!!
Kind Regards,
Fran
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I should say that after performing the VVR installation I swaped tubes by mistake, and put the ez81 in place of an el84 and one of the el84s in the ez81 tube socket.
I think you should get new tubes and try them. IF you know someone who has some EL84's and a EZ81, borrow them for a quick test.
With respect, Tubenit
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Thanks for answering tubenit!
Yeah, I think I'll do that just in case something got broken.
Anyway, I checked the pin correspondence between an el84 and an ez81, and both datasheets and did not expect any harm to the tubes.
I'm not very familiar with datasheet reading though, so who knows...
Hope changing tubes fixes this!!!
Kind regards,
Fran