Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: davidwpack on July 30, 2017, 12:19:33 pm
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Hey there. Just finished a build and doing initial readings. I'm getting high readings on the preamp tubes between plate and cathode on all 3 12ax7 tubes. Between pin 1and 3 I'm getting 172. Between pin 6 and 8 I'm getting 208. For V2 between 1 and 3 is 117. Between 6 and 8 is 80. For the PI between 1 and 3 is 185. Between 6 and 8 is 82. Am I figuring right that I'm running these tubes particularly V1 at about 4-5 watts and about 200%? Should I increase resistor values on the cathode side? Needless to say the amp distorts very early and is very trebly. Any suggestions would be fantastic! Anyways thanks...Dave
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Here's voltage to ground readings. V-1 pin 1=163, pin2 =0, pin3=.93, pin6=194, pin7=0, pin8=1.82. V2 pin1=204, pin2=62.7, pin3=99.5, pin6=298, pin7=203, pin8=208. On V3, pin1=221, pin2=12.2, pin3=39.3, pin6=202, pin7=13.2, pin8=39.4. All heater voltages are fine.
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I don't see anything alarming about those voltage reading. It would be very useful to know what amp you are talking about! Even better to see a schematic.
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It's pretty much a plexi preamp value-wise on the caps and resistors. I used 1 input though and connected V1 to V2 just like the Hoffman 18 watt stout. That connection is the only place I deviated from the plexi. Right now volume at 4 and master at 5 it sounds like a cranked AC30 with the treble at 10 and the bass turned most of the way down. It's hard to get any clean sound at any volume.
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The bass is there and adjustable but not prominent at all, even at 10.
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I rolled back the presence to about 1 and played with the mid-range and it's now sounding pretty decent but still too gainy. I'm thinking of changing the .68 Caps or maybe eleminating them for the gain issue but I think it might decrease my bass frequency too much. I don't know I'll play around and experiment I guess. While I'm at it, I'll post these numbers for the power tubes. They're KT66 tubes. V4 pin 1 and 8=33.3ma, pin3 and 4=463, pin5=-45.3. V5 measurements appear to be identical. These numbers look ok? Anyways, thanks again!...dave
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I think I've got this amp dialed in now. I pulled a .68 cap off V1, put a 12AY7 in V1, 12AX7 in V2, and a 12AT7 in V3. It's cleaned up quite nicely and sounds phenomenal. Lots clean with the volume rolled back and gets gritty with the guitar volume at full. I'm still experimenting with EQ. When I change pickup selections I'm having to re-EQ it seems with a humbucker equiped guitar. My amp seems to love my Mustang though. Anyways, thought I'd update. I hand-drew the schematic for it. I'll try to post it on here later if anyone wants to see it.
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I'll to find a program to do these better later on, but for now I'll try to post my hand-drawn. Hope this works, I've never tried to post pics on here. I'm not the greatest with schematics, if anyone sees errors feel free to tell me. Thanks...Dave
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Couple of pictures.
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And the back. I'm going to build an enclosure when I get back some of my borrowed tools and table saw.
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> post my hand-drawn
Hand-drawn worked from before King Tut's tomb to after Leo Fender.
I like your resistor symbol.
I'm unclear where the 1Meg at the input jack goes. There's a gazillion examples on existing plans. Good draftsmen copy (even trace!) the work of others in little details.
I'd suspect the 1r resistors at KT66 cathodes are or should be separate.
Functionally: the C-R-C-R net from V1b to V2a seems like a lot of bass-cut. (Smells like a reverb driver tuned to avoid boing.) The 470K+47K could logically be 517K, which is just 470K for all practical purpose. But if you are happy, everybody is happy.
Good light on the drawing.
If you prepare drawings a lot, you learn to tune your blueprint machine for whiter whites and darker blues. Using an all-purpose PC image app, they generally aim for "average = 18% grey". This is good for general snapshots but less-good for line drawings.
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Hey, thanks for looking! Yeah, the 1 r,s should be separate. The 1 M goes across the lugs of the input and ground. Glad you caught those, I'll correct them when I get home. The Visio program you guys talk about using for layouts, is that suitable for schematics too? But yeah, the amp does sound great but could use a little more kick on the bass side. I'm not sure where I should start adjusting values yet. But thanks for the suggestions and I welcome any more you might have. Thanks again...Dave.
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Visio does schematics very well. I was using it for schematics many years before I ever did a layout. This pdf was done entirely with Visio (except for the original schematic).
http://sluckeyamps.com/supro/supro.pdf (http://sluckeyamps.com/supro/supro.pdf)
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Nice! I'll definitely check into that this week.