Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on April 05, 2024, 04:05:34 pm
-
PT is putting out high B+ voltage from schematic..? The schematic says 355vdc, l’m know my wall current isn’t 115ac, but l’m getting 394vdc B+. All R’s read specs, except as noted below.
I measure the ac voltage off each rectifier side, getting 353-0-353, schematic calls for 315-0-315. 120ac wall current wouldn’t raise it that much. Original PT.
Also, using a SE old 8 watt OT, 34:1, 8 ohm spk, comes out to about 9500k impedence with old stock 6V6. When I measure for bias, the screen is 373vdc, plate is 368vdc..? 535ohm bias R, voltage across is 23.5. The dissipation is 15.5 watts, cooking.
Only change to schematic is first dropping R to screen is upped from 1k to 2.6k, bias R is 535 from 470ohm. And different OT as mentioned above.
Why is the original PT putting out higher voltage, B+ is 393, (schematic calls for 355, 120 wall V wouldn’t raise that much) and why is the 6v6 screen voltage higher than the plate, when l have a higher dropping R then schematic, 2.6k vs 1k, (The old 8watt OT, l’m using bad as the voltage to plate is way lower than the B+).
Amp sounds great, but 15 watts on 6V6..?
-
5Y3?
-
So 1967... Vibro Champ, unless I'm crazy uses the same PT as Princeton Reverb. Somewhere around the year model that you have, the PT in a Princeton Reverb changed so that it made approx the same B+ voltage with a 5U4 that it previously would have made with a 5AR4. Vibrochamp would have been a 5Y3 either way. So, if you have the newer winding on the PT, your vibrochamp will not have "Blackface" B+, it will have "Silverface" B+. Impossible to know for sure without seeing it first hand, but that's what it sounds like to me.
Those updated voltages don't show up on the schematics, but they are there. Schematics for the princeton reverb show 340-0-340 and claim to make 420 volts whether 5U4 or 5AR4. This does not compute.
On the vibrochamp, the schematic for one of the PT's calls for 315-0-315 and the other says 345-0-345. What a mess.
I'd say you have a 345-0-345. That plus the extra few volts from the wall would get you up to 353-0-353.
Dave
-
plate is 268vdc..? 535ohm bias R, voltage across is 23.5. The dissipation is 15.5 watts,
23.5 / 535 = .044
268-23.5 = 244.5
244.5 * .044 = 10.8W (Including screen, so call it 10.4W)
-
Well, that would make sense, 345 ac is close to 353, and with 120 ac wall, that puts my PT in the ball park. Manufacturing code puts the amp built in 1967, CBS took over in 68. So, if what you say is true, l have a 345-0-345 PT. I have a variac so l can lower the wall to 115. I’ll check, and yes using old stock 5Y3.
That would answer the PT question, but what about the plate being way lower than the B+..?
Only thing between the plate and B+ is the OT. And with the higher node to screen should be lower..?
I could mess with the screen dropping resistor but right now l have 2k7, was 1k. I read here that in this amp better to lower the screen voltage than to up the bias resistor. I’m burning up my 6V6..?
So, this old OT l’m using sounds great but seems to lower the plate voltage “a lot”, causing the dissipation to be too high on the 6V6. The OT came from “something, possibly an organ chassis, there were two of these OT’s, stereo operation and used EL84’s.., but not sure. Could the OT be causing the plate to be much lower than the B+..? If so, what’s the solution..?
-
plate is 268vdc..? 535ohm bias R, voltage across is 23.5. The dissipation is 15.5 watts,
23.5 / 535 = .044
268-23.5 = 244.5
244.5 * .044 = 10.8W (Including screen, so call it 10.4W)
S**t, my bad, not 268, 368, typing mistake, l have fat fingers and these iPhones are an issue for me, :laugh:
-
My guess is that by 1967, the target mains voltage would be 117 not 115, but thats not much of a difference.
Dave
-
After a lot of reading and searching other forums, the BF and SF champs all ran hot, as high as 14 to 15 watts, and higher. Lasted for years and sound great, just like this Vibro Champ with good 60’s tubes. Plus l could use a bucking transformer.
Why my plate voltage is higher than the screen, l don’t know..?
-
Why my plate voltage is higher than the screen, l don’t know..?
Could be that DCR of the OT primary winding is now lower than the screen dropping resistor - 2.7K now?
535ohm bias R, voltage across is 23.5.
original is Rk of 6V6 is 470R, so the you have lowered Ik of 6V6, less load on PS, so > B+ and lighter load to PT will yield > sec. voltage.
--Pete
-
What happens if you change the dropping resistor back to 1K?
-
What happens if you change the dropping resistor back to 1K?
Back to 1k, raises the screen voltage but the plate is still lower by 15/20 volts.
-
Why my plate voltage is higher than the screen, l don’t know..?
Could be that DCR of the OT primary winding is now lower than the screen dropping resistor - 2.7K now?
535ohm bias R, voltage across is 23.5.
original is Rk of 6V6 is 470R, so the you have lowered Ik of 6V6, less load on PS, so > B+ and lighter load to PT will yield > sec. voltage.
--Pete
Thanks, l read a ton about the 1964/and up champs on various forums. Seems with good American made old stock tubes, these amps sound best at 14/16 watts dissipation and up. Sound their best pushing the 6V6GTA (need a good US made one) and the tube will last a long time. HBP, mention to me a while back to keep the 470 biasing R, and increase the first node R, dropping the voltage to the screen. I had a 2.7k, still screen higher than plate. Up it to 5k, bingo 15volts lower on the screen than plate, keeping the drifted bias R that reads 535 ohms.
I started all this as l had a nice old 70’s PR10 from a BFSR, 8 ohms. The original tiny OT in the Champs of that area, just saturated way too much. This old paper wounded SE OT was about twice the size, figured about 8watts and sounds way better, whether or not the DCL of the OT is part of the equation, maybe. Amp is clean up to about 5, each higher number raises the distortion in a good way. Using an old stock 5Y3, wall current 120, tried a variac set to 110/115 and lowered the voltages in proportion. So, at this point my dissipation is 14.9 watts, sounds great, no red plating. I’m happy.
I did notice with tremolo on or off, leaving the intensity at 1, the amp has slightly more gain and volume, I assume this is normal..?
I suggest upping the screen mode to 5/10k from 1k, keeping the 470 bias R, old stock 5Y3 and 6V6GTA and you’re good to go. Upping the bias R just kills the magic in these amps, just lower the screen node to get lower than the plate.
-
Back to 1k, raises the screen voltage but the plate is still higher by 15/20 volts.
This is not a problem. Don't worry. Be happy.
-
Ha, that was my late mother’s favor song, when l’m complaining about something she always sang that song to me.
Yeah, l read that too, not a big deal screen is higher than plate, also read the other way too.
Anyway, l just had to make it lower. BTW, the tone is the same (great) both with the screen higher or lower than the plate, so what’s that tell ya..? :icon_biggrin:
Like my wonderful mother sang to me, “Be Happy, Don’t Worry” when l was complaining about maybe nothing.. :laugh:
-
PT is putting out high B+ voltage from schematic..? The schematic says 355vdc ... but l’m getting 394vdc B+. ...
So 1967... Vibro Champ, unless I'm crazy uses the same PT as Princeton Reverb. Somewhere around the year model that you have, the PT in a Princeton Reverb changed so that it made approx the same B+ voltage with a 5U4 that it previously would have made with a 5AR4. ... will not have "Blackface" B+, it will have "Silverface" B+. ...
Those updated voltages don't show up on the schematics, but they are there. ...
I used to own an all-original 1965 Vibro Champ. The "schematic voltages" are a lie.
Yes, the Champ & Vibro Champ amps use the same PT as the Princeton and Princeton Reverb amps. Which means the PT is under-loaded when powering only a single 6V6 (and no reverb-driver).
Getting 6.3vac from the heater winding in my Vibro Champ required dialing the wall voltage down to 106.9vac. At that point it sent 323-0-323vac to the rectifier, and 366vdc at the 6V6 plate.
Your PT would drop down into a similar range if you pull the wall voltage down closer to 107v. But shy of that approach, you could just increase the value of the dropping resistor between the OT & screen filter cap. Doing so will reduce the screen voltage, and plate dissipation.