Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Big chief on February 07, 2022, 10:22:35 am
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Silverface believe to be 1975 with master volume low volume
Have replaced power tubes and pre amp tubes
Voltage V1 pin1 210 pin6 213 V2 pin1 209 pin6 208 V3 pin1 400 V4 pin1 150 pin6 150
V5 pin1 338 pin 6 337 V6 pin 1 300 pin6 287 V 7-8-9-10 pin 3 429 pin6 424
With volume pots at full open very little sound have used 2 different speaker cabinets 4 ohm
Any help would be great. Thanks
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got a scope?
what's the bias VDC on the grids of the PA tubes?
does the amp have a "pre-amp out" jack?
If so, can you feed that out into another amp/mp3 player?
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Does the issue affect both channels?
What’s the V6 cathode voltage, pin3&8?
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What is the heater voltage at each tube ?
Any mod in the amp ?
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If you could provide the schematic it would be a great help.
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Contact plexi50. He does a lot of repair work in your area.
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Grids V1 pin 2 0 pin7 .03 V2 pin2 0 pin7 .03 V3 pin2 0 pin7 .06 V4 pin2 0 pin 7 .02
V5 pin2 12.9 pin7 12.2 V6 pin2 82 pin7 76
V7 pin8 0 pin3 424 V8 pin8 0 pin3 425 V9 pin8 0 pin3 429 V10 pin8 0 pin3 429
FYI plex is a good friend and helps me
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I do not have a scope no amp out jack
I’m retired and love to work on old amps as I have played old country music for 71 years
Thanks
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https://ampwares.com/amplifiers/fender-silverface-twin-reverb/#
Yesterday ; where is the answer ?
What is the heater voltage at each tube ?
Any mod in the amp ?
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You need a scope and signal generator ; A basic tool to work in a amps.
Without these tools you will only be able to fix the easy problems.
If you don't have these basic tools, you have to compensate with a solid knowledge in electronics
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I have a fair knowledge of tube amps I mostly do the easy stuff
Heater voltages V1 2.3 V2 2.3 V3 3.0 V4 2.3 V5 3.0 V6 3.0
V7 3.5 V8 3.5 V9 3.1 V10 3.5
Does not appear to have any mods this amp has been mistreated imo
I’m trying to help a frien
Thanks
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Voltages
V1 pin1 210 pin 2 0 pin6 213 pin7 .03
V2 pin1 209 pin2 0 pin6 208 pin7 .03
V3 pin1 400 pin2 0 pin6 ? pin7 .06
V4 pin1 150 pin2 0 pin6 150 pin7 .02
V5 pin1 338 pin2 12.9 pin6 337 pin7 12.2
V6 pin1 300 pin2 82 pin6 287 pin7 76
V7 pin3 424 pin6 424 pin8 0
V8 pin3 425 pin6 424 pin8 0
V9 pin3 429 pin6 424 pin8 0
V10 pin3 429 pin6 424 pin8 0
Heater voltages
V1 2.3
V2 2.3
V3 3.0
V4 2.3
V5 3.0
V6 3.0
V7 3.5
V8 3.5
V9 3.1
V10 3.5
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I have a fair knowledge of tube amps I mostly do the easy stuff
Heater voltages V1 2.3 V2 2.3 V3 3.0 V4 2.3 V5 3.0 V6 3.0
V7 3.5 V8 3.5 V9 3.1 V10 3.5
Does not appear to have any mods this amp has been mistreated imo
I’m trying to help a frien
Thanks
They are low ; too low .
Voltage reading are from the ground ?
You should read 6.3 volts between pins 5/4 and 9 on preamp tubes
From ground you must have 6.3 / 2 =3.XX not 2.3
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Thank you uki for doing this, much easier to read now! :icon_biggrin:
Voltages
V1 pin1 210 pin 2 0 pin6 213 pin7 .03
V2 pin1 209 pin2 0 pin6 208 pin7 .03
V3 pin1 400 pin2 0 pin6 ? pin7 .06
V4 pin1 150 pin2 0 pin6 150 pin7 .02
V5 pin1 338 pin2 12.9 pin6 337 pin7 12.2
V6 pin1 300 pin2 82 pin6 287 pin7 76
V7 pin3 424 pin6 424 pin8 0
V8 pin3 425 pin6 424 pin8 0
V9 pin3 429 pin6 424 pin8 0
V10 pin3 429 pin6 424 pin8 0
Heater voltages
V1 2.3
V2 2.3
V3 3.0
V4 2.3
V5 3.0
V6 3.0
V7 3.5
V8 3.5
V9 3.1
V10 3.5
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Do you know V7,8,9,10 pin 8 are ground ?
0 volts is right
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Big chief, you need to measure the heaters across their pins, 4/5 - 9, 2 - 7 and not 1 meter probe to a heater pin and the other meter probe to ground.
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Voltages
Heater voltages
V1 2.3 = 4.6 heater voltage :BangHead:
V2 2.3 = 4.6
V3 3.0 = 6
V4 2.3 = 4.6
V5 3.0 = 6
V6 3.0. = 6
V7 3.5 = 7 !!!!
V8 3.5 = 7 !!!
V9 3.1 = 6.2 Good
V10 3.5 = 7
If you read 7 volts at V8 , you must read 7 volts at V9 not 6.2 :w2:
Tubes are wired in parallel
Something wrong with your test meter or amp wiring or.......... ?
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Big chief, you need to measure the heaters across their pins, 4/5 - 9, 2 - 7 and not 1 meter probe to a heater pin and the other meter probe to ground.
That is what I wrote 1 hour earlier........
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Pins 4-5 on this amp are tied together
At pilot light from ground on each side 3.1 Across the pilot light 6.2
Pin 8 to ground 0
I have check plate resistors none out of specs next check coupling caps for dc current I guess
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Pins 4-5 on this amp are tied together
At pilot light from ground on each side 3.1 Across the pilot light 6.2
Pin 8 to ground 0
I have check plate resistors none out of specs next check coupling caps for dc current I guess
Yes 4 and 5 are always wired together, that is what we mean when we wrote 4/5 and pin 9
Read with voltmeter at pin 9 ansd pin 4 or 5 . you should read 6.2 as pilot light
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uki wrote ;
If you could provide the schematic it would be a great help.
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....Across the pilot light 6.2
That fine, but because you had lower readings before on the 1st/2nd preamp tubes, best measure that 1st preamp tube again across pins 4/5 and 9.
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Found some frayed wire going to replace all heater wire then recheck voltages
Thanks for all your time and help
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Repaired heaters wires to all tubes
Have 6.2 on all heaters set power tube bias at -49 all at once all tubes lost power but pilot light stayed on Now don’t know what to do
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all tubes lost power
as in the filaments went out or the B+ went to zero?
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I found the problem loose heater wire my fault.
However I have dc voltage through the coupling caps should I replace them .1uf
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However I have dc voltage through the coupling caps should I replace them .1uf
Those caps are supposed to have dc on each side.
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I found the problem loose heater wire my fault.
Congrat !
Answer #3 february 7
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Have checked ohm’s on all resistors none seam to be more than 10%
Rechecked voltages
V1 pin1 219 pin6 227 V2 pin 1 222 pin6 228 V3 pin1 414 pin6 412 V4 pin 1 230 pin6 232
V5 pin 1 353 pin6 353 V6 pin1 312 pin6 301
V7 pin3 439 pin6 403 V8 pin3 439 pin6 402 V9 pin3 434 pin6 434 V10 pin3 435 pin 6 433
6.2 to all heaters
Power tubes are new and checked pa tubes test good on my tube tester have replaced some and moved around Chopped stick board no loose connections What am I missing still very low volume
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Many tube tester are not reliable, best is to pu know good preamp tubes
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Examine the speaker jacks. Is the plug in the correct jack? Do you have the simple two lug jack for the external speaker? Or is it the more complicated jack with transfer circuit switch contacts as found in later TRs?
You still have not provided a correct schematic for your amp. There are more than one TR circuit with a MV and we cannot guess which one based on the little info you have provided.
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You still have not provided a correct schematic for your amp. There are more than one TR circuit with a MV and we cannot guess which one based on the little info you have provided.
Third time we ask :BangHead:
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I’m using schematic Twin reverb Aa270 layout
I search this web site a d printed the schematic that I’m using
I have not been able to find the exact schematic for this amp yet and I have searched all I know
Have used 2 different speaker cabinets with 4 ohm and have checked and cleaned the 2 jacks that are on the chassis I’ll try my best to take photo of the chassis that I’m working on
This amp is a mid 70’s with master volume
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I’m using schematic Twin reverb Aa270 layout
Wrong layout ; no master volume
Did you look to my link ? Answer #8
All are there.
Can you read schematic ?
Layout is not good to trouble shouting.
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Yes I can read schematic I hope I answered your # 8
I am still looking for schematic with master volume now I’m trying to send photo of chassis
Some times I get a little confused
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Please post more pictures from the other side of the chassis, as well front and back faces, someone might spot the schematic for you with that info !
Oh and about the heater wires, make sure the same wire is in the same pin number for all sockets, unless the schematic says otherwise.
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Yes I can read schematic I hope I answered your # 8
I am still looking for schematic with master volume now I’m trying to send photo of chassis
Show us the right schematic.
Schematic is the first thing you need before working in a amp .
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I am still looking for schematic with master volume now I’m trying to send photo of chassis
Look in Hoffman's schematic library. It's there. Does your amp's MV have a push/pull switch on it? If so, look for that schematic. If not, look for the MV without the push/pull switch.
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The 470ohm unbypassed V3 cathode resistor shown in the photo indicates that this may be the appropriate schematic https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_100_schem.pdf
The previous version without the pull boost has a bypassed 2k2 V3 cathode resistor https://el34world.com/charts/Schematics/files/Fender/Fender_twinreverb_sf_100_mv_noboost.pdf
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You have a very good eyes
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I was getting desperate trying to find something to move this along! :laugh:
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I was getting desperate trying to find something to move this along! :laugh:
:occasion14:
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More Photos
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More Photos
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The resolution of those photos is inadequate for them to be much use. We need to be able to zoom in and get detail, not mush.
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hope these photos are better
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one more time
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What is that pot ?
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That pot hum balance for power tubes to the best of my knowledge
Could I possibly have a weak out put or choke problem it seams no matter what I do the volume does not change I have replaced every tube even power tube sockets
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That pot hum balance for power tubes to the best of my knowledge
Could I possibly have a weak out put or choke problem it seams no matter what I do the volume does not change I have replaced every tube even power tube sockets
Without the right ( and basic amps repair ) tools ; scope and audio generator....... good luck
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That pot hum balance for power tubes to the best of my knowledge
It looks like it’s connected to V2 but I’m having trouble tracing it within the rats nest. For what it’s worth the SF Fenders I’ve worked on that had hum balance pots did not look like that, but I’m not familiar with the variation you’re working on.
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That pot hum balance for power tubes to the best of my knowledge
It looks like it’s connected to V2 but I’m having trouble tracing it within the rats nest. For what it’s worth the SF Fenders I’ve worked on that had hum balance pots did not look like that, but I’m not familiar with the variation you’re working on.
I agree.
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Thanks to everyone that took your time to help me it was greatly appreciated
Amp headed to plexi50 Monday
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Thanks to everyone that took your time to help me it was greatly appreciated
Amp headed to plexi50 Monday.
Please let us know what plexi50 finds. :icon_biggrin:
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Big cheif if you still need the schematics and lay out I have it, If no one can cure your SFTR 75" I will be happy to help , They are my favorite amp, many misleading speaches are repeated about these amps,and they can never be a black face, however a few simple configs will have that amp stomping everything around, Ive been playing them like this for over 40 years . Ill be happy to help if needed. marcus
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Big cheif if you still need the schematics and lay out I have it, If no one can cure your SFTR 75" I will be happy to help , They are my favorite amp, many misleading speaches are repeated about these amps,and they can never be a black face, however a few simple configs will have that amp stomping everything around, Ive been playing them like this for over 40 years . Ill be happy to help if needed. marcus
Like blackface these amps will always works and play nice in 40 years and more with basic maintenance