Plate Voltage supply values too High

[dr.earlfunkton]

I am currently in the testing stage of an amp I am building and noticed that the voltage supplied to all of the tube plates are too high.

I built the phase inverter and power section based off a Bogen Challenger CHB-50; the schematic can be seen below.
The Preamp section is based off of a Fender AB763 Twin Reverb schematic, also included below.
I am trying to get the supply voltages to what are detailed in the Bogen schematic.

The voltages I am getting from the power filter section (as well as the target voltages) are as follows:

Target                   Actual
445V                     504V
420V                     497V
320V                     474V
300V                     461V

The voltages I am getting at the tube plates are as follows:

      Target                     Actual
V1   300V/320V            218V/107V
V2   xxxx/320V             xxx/0.3mV
V3   320V                     477
V4   420                       501
V5   420                       501

Note: I am only using one half of the 12AX7 in the V2 position so one half isnt being supplied with B+.

For some reason the supply voltages are (for the most part) way too high. I have built this amp from scratch (except for the transformers and chassis) so all the parts are new and of high quality.
I noticed on the Bogen Schematic the values listed are for a 117VAC supply. I measured my line voltage and got 119VAC. I dont think 2VAC would cause for such a high voltage supply coming from the secondaries of the PT.

A second note, the filament supply was only reading about 5.3VAC on each position.
Should I try adding higher value resistors on the filter cap can or possibly try and lower the line voltage coming into the amp?

[dr.earlfunkton]

AB763 Schematic

[pompeiisneaks]

First off, I didn't seem to see if these are with tubes in or not?  If not, they'll drop pretty significantly with tubes in. 

If so, dropping resistors will help some, but dropping that much is often better with some kind of mosfet/zener diode setup on the center tap of the transformer.  There are some good howto's on that out there, I'd have to hunt for one, but someone may have one more readily available.  The one I've read up on is the last entry here:
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

but I've seen somewhere a nice article where someone shows the simple diode list that are good high voltage droppers, and which to use where and such.  I don't know where to find it though.

~Phil

[dr.earlfunkton]

First off, I didn't seem to see if these are with tubes in or not?  If not, they'll drop pretty significantly with tubes in. 

Ah, forgot to say haha. No, there were no tubes in the amp when I was testing voltages. If they will drop, then the voltages at V1 and V2 will be far too low for ideal operation as they are already at that point.
Any idea for a fix there?

[labb]

No tubes installed=no current flow=no voltage drop.


be sure to check your -bias voltage before you install the tubes and power up.

[sluckey]

Ah, forgot to say haha.

Ha ha! Plug the tubes in and reevaluate.

[John]

      Target                     ActualV1   300V/320V            218V/107VV2   xxxx/320V             xxx/0.3mV


Surely that's not right?

[pdf64]

It may be that the Bogen amp mentioned has been used as a doner. If so, it looks to have been built for 6EU7, 12AX7 and 6C4 small signal tubes. Maybe that has caused some confusion?

[jjasilli]

Posting schematics as examples of an amp that you didn't build is a recipe for confusion.  This Reply is based on a true story. 


Are you using the Bogen PT or not!?!   
That was a rhetorical question.  Please don't answer it verbally.  Instead, post your actual schematic.


It may be that the Bogen amp mentioned has been used as a doner.
That's a good guess, but why in the name of Sam Hill are we guessing?  It's a waste of time, energy and brain power.  The poster should provide this basic info. 


Anyway the Bogen was no doubt expecting a wall supply of 110 or 115 VAC.  With today's 120 supply, that alone accounts for much of the difference, with the remainder within the margin of error.  Whether or not this comment applies to your amp is indeterminate, unless and until you post the actual facts, and not some fairy tale about other amps that your amp is "based on".

[92Volts]

If you're missing a capacitor from one of the tone stacks, this would provide a path for DC current to ground through the pots in the tonestack.

With no tubes you'd expect the voltages to be high and similar-- nearly 500v everywhere. The super-low voltage at V1 suggests current is flowing somewhere it shouldn't, and the uneven voltage between each side of V1 suggests a tonestack or other audio circuit problem, rather than a power supply problem.

[dr.earlfunkton]

I realized I forgot to hook up the plate voltage to pin 6 of V1, so I fixed that last night. I remeasured the voltages after installing the tubes and they seem slightly better but still just a touch too high while V1 and V2 seem low.

Here are the new plate voltages after I installed the tubes:
V1A   165V
V1B   198V
V2A   208V
V3     328
V4     356
V5     349

pdf64: I wired V1 and V2 both for the pinout of 12AX7s as per the fender schematic provided. V3 (the 6C4) was wired as it should have been as per the bogen schematic provided.


jjasilli: I did post the schematics I used. I guess I shouldnt have used the word "Based". I built the preamp to the exact specifications of a single channel (input, tone stack, three 12AX7 halves leading up to but not including the phase inverter) of the AB763 schematic. Where I change over from the fender schematic is at the .1uF capacitor off of the plate of the 2nd half of the 7025 that would be for the reverb (I didnt build the reverb network in this amp). Everything after that (PI to output section and power supply) was constructed to the exact specifications in the Bogen Schematic. I circled the spot on both schematics where the fender tone stack meets the bogen power section etc. Im new to this hobby and learning as best as I can through this amp build. Id appreciate any helpful insight you might have but being a dick and belittling me isnt helping anybody.

[dr.earlfunkton]

Ill draw up a schematic tonight of what my amp looks like to ease any confusion so people dont have to look between two schematics

[dr.earlfunkton]

And the other one

[labb]

The Fender PS node for the pre amp tubes puts 410 VDC to the plate resistors. The Bogen puts 300 VDC to the plate resistors. Assuming you wired it that way. Check what your bias voltage is at pin 5 of V4 and V5. What is your rectified voltage at the first filter cap of the ps where it shows 445V.

[sluckey]

jjasilli...    Id appreciate any helpful insight you might have but being a dick and belittling me isnt helping anybody.

Hey ace! Calling someone a dick is likely to get your problems resolved real quick!

[jjasilli]

I'll take the heat, but personal insults are not cool around here.


Before I post for help, I consider it my responsibility:


1.  to have and to post an actual schematic of the amp, device or circuit in question.  If I can't find one after best efforts at known websites, a google search, Forum search, etc.; then I'd post for help to find the schematic.  If I modded an amp, I consider it my job to draw a new schematic.  I consider this to be a part of the trouble shooting process.
2.  to search this Forum, other Forums and google to see if there's already an answer.
3.  to go through logical, step-by-step troubleshooting procedures using published troubleshooting flowcharts or checklists.

A pet peeve of mine is when people fail or refuse to do these things.  Many members of this Forum are selflessly willing to go to any length to help-out.  But I consider it to be an imposition when basic information is withheld to make this an effective and straightforward process.  Guys go inadvertently barking up the wrong tree trying to be helpful, and a thread needlessly becomes a tangled mess.

[jjasilli]

I remeasured the voltages after installing the tubes and they seem slightly better but still just a touch too high while V1 and V2 seem low.Here are the new plate voltages after I installed the tubes:V1A   165VV1B   198VV2A   208VV3     328V4     356V5     349

Good news:  There is nothing wrong with these plate voltages.  The power tubes are well within 6L6 spec, and lower than Bogen spec.  For guitar, you might prefer higher preamp plate voltages, per Fender specs.  This is done by lowering the value of the dropping resistors in the B+ rail.  Use Ohm's Law to compute an estimate of new R values.  Post if you want help with that.

You may also want to check your power tube bias. 

[pdf64]

...I built the preamp to the exact specifications of a single channel (input, tone stack, three 12AX7 halves leading up to but not including the phase inverter) of the AB763 schematic. Where I change over from the fender schematic is at the .1uF capacitor off of the plate of the 2nd half of the 7025 that would be for the reverb (I didnt build the reverb network in this amp). Everything after that (PI to output section and power supply) was constructed to the exact specifications in the Bogen Schematic. I circled the spot on both schematics where the fender tone stack meets the bogen power section etc. ...

Please note that the Fender TR AB763 schematic provided has 2 channels; until now, no one other than yourself has known that your 'based on' has only included part of one of them.

Only you know what you've got in front of you, everyone else is having to rely on your communication skills.

Can you see how dreadfully vague and unhelpful you are being?

{edit- untangled quote -PRR}

[dr.earlfunkton]

A pet peeve of mine is when people fail or refuse to do these things.  Many members of this Forum are selflessly willing to go to any length to help-out.  But I consider it to be an imposition when basic information is withheld to make this an effective and straightforward process.  Guys go inadvertently barking up the wrong tree trying to be helpful, and a thread needlessly becomes a tangled mess.

Please note that the Fender TR AB763 schematic provided has 2 channels; until now, no one other than yourself has known that your 'based on' has only included part of one of them.

Only you know what you've got in front of you, everyone else is having to rely on your communication skills.

Can you see how dreadfully vague and unhelpful you are being?

Sorry y'all, I get I wasn't giving all the information needed to get help on this. Again, still pretty fresh to this stuff and especially communicating about it on a forum. Sometimes I have a hard time finding the right vocabulary to explain what im talking about when it comes to amp building/repair.

The Fender PS node for the pre amp tubes puts 410 VDC to the plate resistors. The Bogen puts 300 VDC to the plate resistors. Assuming you wired it that way. Check what your bias voltage is at pin 5 of V4 and V5. What is your rectified voltage at the first filter cap of the ps where it shows 445V.

For V4 the bias is -37.01V and V5 is -35.15V

The voltage coming off of the first filter cap is 373V.
The voltages coming from the cap can are as follows:

Schematic                         Actual
420V                                 354V
320V                                 328V 
300V                                 327V

Good news:  There is nothing wrong with these plate voltages.  The power tubes are well within 6L6 spec, and lower than Bogen spec.  For guitar, you might prefer higher preamp plate voltages, per Fender specs.  This is done by lowering the value of the dropping resistors in the B+ rail.  Use Ohm's Law to compute an estimate of new R values.  Post if you want help with that.

What range of voltage on the plate would I want to be in for fender specs? I see that the maxium plate voltage for a 12AX7 is 330V according to the RCA receiving tube manual.
I measured the DC current at pins 1 & 6 of V1 and pin 6 of V2, so after the dropping resistors. Is this the right point to measure the current draw?
The measurements I got were:

V1,Pin 1        51.3mA
V1,Pin 6        62.0mA
V2,Pin 6        62.5mA

This seems really high to me, the RCA manual says the max is 1.2mA for the combined plates.

[John]

I measured the DC current at pins 1 & 6 of V1 and pin 6 of V2, so after the dropping resistors. Is this the right point to measure the current draw?The measurements I got were:V1,Pin 1        51.3mAV1,Pin 6        62.0mAV2,Pin 6        62.5mA

I suspect your math is flawed in some manner. To measure the current at the plate, measure voltage across the plate load resistor (typically that is a 100K ohm) Divide that voltage by the resistance, that is your current. So, if you're dropping (measuring) say 150 volts across a 100K ohm resistor, that is .0015 or 1.5mA.


Just an aside, we rarely use the "amperage" setting on our meters. Nearly always we measure resistance (with the amp turned off!) then voltages with the amp on, and do the math. Ohm's Law gives the answer to nearly every question. :)

[dr.earlfunkton]

I measured the DC current at pins 1 & 6 of V1 and pin 6 of V2, so after the dropping resistors. Is this the right point to measure the current draw?The measurements I got were:V1,Pin 1        51.3mAV1,Pin 6        62.0mAV2,Pin 6        62.5mA

I suspect your math is flawed in some manner. To measure the current at the plate, measure voltage across the plate load resistor (typically that is a 100K ohm) Divide that voltage by the resistance, that is your current. So, if you're dropping (measuring) say 150 volts across a 100K ohm resistor, that is .0015 or 1.5mA.


Just an aside, we rarely use the "amperage" setting on our meters. Nearly always we measure resistance (with the amp turned off!) then voltages with the amp on, and do the math. Ohm's Law gives the answer to nearly every question. :)

Ahhh I see. I think I was using Ohms law the other night and got more reasonable numbers. Ill check it again, thanks :)

[jjasilli]

What range of voltage on the plate would I want to be in for fender specs?
You posted a Fender schematic.  The preamp plate voltages are stated thereon -- about 250VDC or more.

[dr.earlfunkton]

What range of voltage on the plate would I want to be in for fender specs?
You posted a Fender schematic.  The preamp plate voltages are stated thereon -- about 250VDC or more.

Oh wow, I cant believe I was confused. Going between the Bogen and Fender schematics I didnt realize that on the Bogen schematic, it lists the voltage being fed to the plate resistor, where the Fender schematic lists both the supply rail voltage before the plate resistors as well as the required voltage at the plates. Duh.

BTW!

Finally got the schematic fixed to what my amp looks like currently. Let me know if something seems off or confusing!

[sluckey]

V1 pin 2 needs a resistor to ground. 1M is typical and can be located directly on the input jack.

V2 pin 7 also needs a resistor to ground. 470K or 1M are typical.

[dr.earlfunkton]

V1 pin 2 needs a resistor to ground. 1M is typical and can be located directly on the input jack.

V2 pin 7 also needs a resistor to ground. 470K or 1M are typical.

For V1 Pin 2, can I just place a resistor from the Tip lug to the Sleeve lug, or do I need to wire it to a separate grounding point off of the jack?

Also for V2 Pin 7 should I solder it right from the pin to ground?

[sluckey]

A 1M resistor is typically mounted directly on the input jack. For V2-7 place the resistor in a convenient location. That may be directly on the tube socket or on a terminal strip or a turret/eyelet board. Just depends on your build style. I like using turret boards so I would put that resistor on the board.

[jjasilli]

Oh wow, I cant believe I was confused.
Hence the value of a proper schematic to start with.  I rest my case.


In addition to the recent comments made by sluckey: on the schematic, correct voltages should be stated at each relevant tube point, such as: plate; screen, if any; cathode (if self biased).  Alternatively, a voltage chart may be used.  Your voltage chart is in doubt as it continues to list the 6EU7 which I think is not there any more.


Once you have a full-fledged, proper schematic you then have a clear idea of your amp.  If your amp isn't working right, like improper voltage on a tube plate -- paste a copy of the schematic above your workbench.  Verify the connections in your amp visually, and physically with a chopstick.  Use a highlighter pen to mark over the lines in the schematic to show that the connection is good.  Rinse & repeat.