Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: theundeadelvis on November 02, 2011, 11:38:06 pm
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Hello again guys! As mentioned in a previous thread, I'm in the process of building a new amp. I'm trying to keep it simple, but I'm interested in using the VVR from Hall. I've been reading all the posts here (and all over the web), and it seems that most people prefer scaling the power amp or power amp and PI over scaling the entire amp. I've read the technical reasons why many prefer this, but I'm not sure how this translates into what I'm actually going to hear. Could someone explain (in layman's terms) how these methods sound different (I'm sure it depends a lot on the particular amp too)? I'm not looking for bedroom volumes, I just want to be able to knock it down a few db's and affect the tone as little as possible.
My concern at this point is, I've already populated my turret board, and I'm not sure how easily I could include the additional parts recommended for PA and PI scaling (extra filtering, diodes) and a PPIMV, so the simplicity of whole amp scaling and leaving out the master volume is appealing. BUT, if I'm going to lose the character of my amp in the process, then it's not worth the trade-off.
I'm attaching the schematic and layout of the amp in case anyone has any other suggestions. Maybe VVR isn't the way to go on this particular build? Maybe just a PPIMV or even a crossline master (keeping it super simple).
As always thanks a lot, in advance, for your input!
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Scaling the whole amp, the power section + PI or only the power section depends on the amp topology and what you're looking for tone-wise. A scaled stage will distort sooner.
Your project uses a cathode-biased output section with a Schmitt PI, I'd scale both sections and add a type II/LarMar PPIMV - a MV is must in most scaled amps.
A few other suggestions:
- Use a 34K input R. The two V1 sections in // show a doubled Miller capacitance, no need for 68K here.
- Lower plate load for V1: 100K shared between two sections is akin to 200K per section. Lotsa gain, maybe too much. You can use a 56K 2W resistor instead.
- Higher V1 cathode bypass. Using a 0µ68 cap could lead to too bright a stage, 2µ2 could sound better here.
- If I read correctly the low-res schemo the PT puts out 300 -0 - 300 V. Consider using two 1N4007s in series and a 500V cap for C1.
- Use a 1K 3W resistor in the supply between plates and screens. A 2W model could overheat.
- Consider splitting the KT66s' cathode load and use a Geezer switch instead of going VVR.
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Thank you FYL!
I forgot all about the Geezer Switch. I remember following the original thread about it, and I always wanted to implement it, but it slipped my mind. I wasn't able to dig it up searching "Geezer Switch," but I think I may have the schems saved somewhere on my server.
I think I understand all of your suggestions, except the series diodes? I'm not sure how you mean to use these (or what they function as)? Also, all of my filter caps are 500v (I'm a "better safe than sorry" kinda guy).
Thanks again for taking the time to make these suggestions! I appreciate your input!
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You're using a full-wave recto. The diodes will see close to 1KV, the max rating for a 1N4007. Most will work reliably, but, just in case, double them (in series).
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I have a fairly high gain non master volume ( trainwreck type) amp that I scaled with a vvr. did the whole amp. love it! I goes from about 5 watts to about 20 watts and its extremely handy to have it. Tone gets slightly mushy when i am really turning it down but not even noticeable down halfway. Used the Hall vvr and it has functioned perfectly, was easy easy easy to install. For what thats worth, good luck!
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OK, I understand now. Thanks again FYL!
zambo - thanks for the info. I may still consider it an option, since scaling the entire amp is fairly unobtrusive. I'd also like to experiment with the Geezer switch, but I've haven't found the thread yet, and my server hard drive just crapped out on me. :sad2:
Thanks again!
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Hi Theundeadelvis
are you looking for this ?
http://www.el34world.com/Forum/index.php?topic=4401.0 (http://www.el34world.com/Forum/index.php?topic=4401.0)
Kagliostro
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Bingo! Thank you kagliostro!
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FYL - quick question, you mention 34k input resistor, because of the paralleled V1. If I use the switch that's in the layout, that gives the option of paralleled or a single triode, should I stick with the 68k or still use your recommended 34k? I may not use the switch, as I'm guessing I may prefer it paralleled and leave it, but I am curious nonetheless. Thanks!