5F10 Schematic question

[NTBluesGuitar]

Well, I haven't tried a 6AT6...I just decided on the 6AV6 based on spec sheets and availability.

From that alone, my impression is the difference between the two would be similar to the difference between a 12AX7 and 12AT7.

[Danelectron]

Well, it's been a while but I finished my 5f10. I have a bit of troubleshooting to do. My voltages are running a bit low, and the power tubes are apparently drawing too much current. The plates are glowing reddish. I tried swapping the rectifier tube with the same result. It sounds good though. I don't have much more to add at this point. I'm a bit stumped with the low voltages.

[tubeswell]

My voltages are running a bit low, and the power tubes are apparently drawing too much current. The plates are glowing reddish. I tried swapping the rectifier tube with the same result. It sounds good though. I don't have much more to add at this point. I'm a bit stumped with the low voltages.

Hi Danelectron

Red plates is too hot for sure

What are the DC idle voltages? (B+, Plates, screens, grids)

Have you worked out/measured the tube current?

[Tom Phillips]

One likely cause is that your bias is incorrect. If too much bias current is flowing it will drag down your B+ voltage AND cause excessive plate dissipation which could result in red plating. As a starting point, measure the 6V6 grid voltage. Is it close to the -27V shown on the Fender schematic? Problems could be due to a bad bias circuit, bad caps between the phase inverter and the power tubes. i.e. leaking DC or a bad (runaway) power tube You can start by measuring the grid and plate voltages on the power tubes with the tubes pulled removed and post the results.
Regards,
Tom

[GibsonLover]

If you like the Harvard, you'd probably like the Princeton (12AX7's). You might consider the 6G2 Brown Princeton with the 5D3 Tweed Deluxe output section. Just loose the Trem stage and use the triode for the PI.

Or... the Champ AA764 (with Blackface tone stack) preamp and the 5D3 output. More versitile, yet simpler. So many ways...

[Danelectron]

Tom, Tubeswell and All,

I've had a busy summer, but got around to checking some voltages. All these measurements were to gnd.
(edit) the columns got screwed up in posting so try to read down each tube number, sorry

6V6-1 6V6-2 12AX7 6AT6
Pin 2- 246v Pin 2- 238v Pin 1- 136v Pin 1- 1.3v
Pin 3- 240v Pin 3- 238v Pin3- .77v Pin 2- 1.3v
Pin 4- 252v Pin 4- 252 Pin 4,5- 219v Pin 3- 219v
Pin 5- 1.04v Pin 5- .62v Pin 6- 166v Pin 4- 216v
Pin 6- 1.14v Pin 6- .695 Pin 7- 14.7v Pin 7- 115v
Pin 7- 221v Pin 7- 221 Pin 8- 38v

I didn't have alot of time to troubleshoot this before work, but made some additional measuements as follows:
Filter cap 1- 262v
Filter cap 2- 252v
Filter cap 3- 203v
rectifier tube pins 4 and 6- 637 vac, pins 2 and 8- 271.4vdc ( same readings on two different rectifier tubes)
diode junction at the two 220K resistors- 17.7v

Tom, I will pull the power tubes and check the power tube sockets again. I noticed a slight scent of a hot component, but this may just be some burn in. It was very slight.

Thanks to you guys for your attention.

[Danelectron]

GibsonLover,
I did build a 5f2 (for my first amp) and a 5E3 (and do really them both), but combining the two ( 6G2 and 5D3 ) may have to wait til I learn a bit more about each section. I very well may build the 6g2 though for the experience then maybe do your mod later. The first two went together without a hitch, but I'm kind of glad the 5F10 is giving me something to delve into. Troubleshooting is where the real education comes in. I just feel bad that I'm a bit lost. Thanks, Dan

[tubeswell]

Tom, Tubeswell and All,

I've had a busy summer, but got around to checking some voltages. All these measurements were to gnd.
(edit) the columns got screwed up in posting so try to read down each tube number, sorry

6V6-1 6V6-2 12AX7 6AT6
Pin 2- 246v Pin 2- 238v Pin 1- 136v Pin 1- 1.3v
Pin 3- 240v Pin 3- 238v Pin3- .77v Pin 2- 1.3v
Pin 4- 252v Pin 4- 252 Pin 4,5- 219v Pin 3- 219v
Pin 5- 1.04v Pin 5- .62v Pin 6- 166v Pin 4- 216v
Pin 6- 1.14v Pin 6- .695 Pin 7- 14.7v Pin 7- 115v
Pin 7- 221v Pin 7- 221 Pin 8- 38v

I didn't have alot of time to troubleshoot this before work, but made some additional measuements as follows:
Filter cap 1- 262v
Filter cap 2- 252v
Filter cap 3- 203v
rectifier tube pins 4 and 6- 637 vac, pins 2 and 8- 271.4vdc ( same readings on two different rectifier tubes)
diode junction at the two 220K resistors- 17.7v

Tom, I will pull the power tubes and check the power tube sockets again. I noticed a slight scent of a hot component, but this may just be some burn in. It was very slight.

Thanks to you guys for your attention.

Pin 5 of each 6V6 should be about -27VDC at idle. - similar for the junction of the 2 x 220k grid load resistors. -17V means that the 6V6s will be running on the hot side.

What VAC is the bias supply tap off the PT? I think the stock 5F10 PT delivers 50VAC, but you better check your one to be sure. (If it is less than 50VAC, then that would explain why you aren't getting enough bias voltage). The other reason could be that your bias supply voltage divider is wrong.

50VAC should work with 6k8 and 56k in the bias supply voltage divider, but its better if you make the bias supply adjustable. Replace the 56k with a 50k pot wired as a variable resistor in series with a 39k resistor on the pot ground leg. Wire the pot wiper to one of the other pot's lugs - to ensure that if the wiper contact fails you still have a bias supply voltage.

[Danelectron]

Tubeswell, I'm using Allen Heyboer transformers recommended by NTBluesGuitar. He set his up with adjustable bias which may be the key with this power transformer. I will try that first. I was wondering about the voltage divider in general since all the voltages are a bit low, but I imagine the most critical is the bias divider. Thanks, Dan

[Tom Phillips]

In addition to what tubeswell said I'll add that it appears that there is something seriously wrong with the bias supply.
You read -17V at the diode junction at the two 220K resistors but measure 1.04v and .62vat the 6V6 grid pin. those voltages should be essentially the same. i.e. the grid should be at the same idle voltage as that at the junction at the two 220K resistors. Take out the power tubes and measure again as you planned. Don't put the power tubes back in until the problem is found and corrected. I still suspect a leaky coupling cap. (The .1 uF cap off the 12AX7 plate is most suspect)
Regards,
Tom

[Tom Phillips]

One more thing.
You posted that you measured 219V on pins 4 & 5 of the 12AX7 which is the heater connection. That's not at all right. Time to do some double checking of your wiring and measuring.
Tom

[Danelectron]

Tom, I will have to remeasure the 12AX7. I may have wrote that down wrong. I was in a hurry to get on the road. I imagine 219 on the heater would have lit up the room. I did measure the bias supply and the transformer is providing about 53 volts. it drops to -19 past the 6.8k resistor and down to -17= past the diode. I need to get a 50k pot to set up the adjustable bias, but will pull the existing components and test them. I did notice the difference on pin 5 of the 6v6s. I will check the 1.5k resistors there as well as the coupling caps. I did go over the wiring per the schematic, but it wouldn't be the first time I have missed something twice. I highlight the wiring on the schematic as I complete wire runs, but if I get called away from it for any reason I may miss things. Payback for squandering my youth I imagine. I did pull the power tubes today and took the following measurements on the 6V6s:

6V6-1 Pin 2- 219v Pin 3- 444v Pin 4- 443v Pin 5- 19v Pin 6- 19v Pins 7 and 8- 0
6V6-2 Pin 2- 219v Pin 3- 444v Pin 4- 442v Pin 5- 19v Pin 6- 19v Pins 7 and 8- 0


Thanks for your help, both of you. I'm going to try to get to this on Sunday. It is surprising that it plays pretty well and the tone control has great range in spite of all this. I only strummed it a bit until I saw the glow and haven't had it on for more than a few minutes to do the voltage measurements since. Thanks! Dan

[tubeswell]

took the following measurements on the 6V6s:

6V6-1 Pin 2- 219v Pin 3- 444v Pin 4- 443v Pin 5- 19v Pin 6- 19v Pins 7 and 8- 0
6V6-2 Pin 2- 219v Pin 3- 444v Pin 4- 442v Pin 5- 19v Pin 6- 19v Pins 7 and 8- 0

The VAC between Pin 2 and Pin 7 should be ~6.3VAC. Don't worry about trying to measure DC (In fact Pin 2 shouldn't have 219VDC on it - that is a heater pin). How are your heaters wired?

Have you got a grid stopper resistor between pins 5 and 6? - whatever - Pin 5 is still not negative enough. It should be around -27VDC.

[Danelectron]

Tubeswell, I double checked the heaters on the 12AX7 and found the 219 was mvDC. I switched to AC and it was 6.3. Same mistake on the 6AT6. I do have a grid stopper between 5 and 6. Didn't get any further today. They have me working all weekend so I will get into it more tomorrow night. I'm sure I made the same mistake on the pin 2 measurements when I pulled the 6V6s so I will recheck them also. Should pins 3 and 4 read as high 444vdc to ground with the tubes removed? It seems like it shouldn't be any higher than 262v and 252v as read from the filter caps. I didn't check the filter cap voltages when I pulled the power tubes. I apologize for the misinformation. I need to just slow down and take some time to concentrate on this process. Thanks.

[tubeswell]

Should pins 3 and 4 read as high 444vdc to ground with the tubes removed? It seems like it shouldn't be any higher than 262v and 252v as read from the filter caps. I didn't check the filter cap voltages when I pulled the power tubes.

Yes its normal for the B+ side voltages in 'unloaded' circuits to be higher. Pins 3 (plate) and 4 (screen) will read right up to the B+ level without a tube installed. The tubes 'pull down' the 'pin' voltages because when the tubes are installed, current is flowing through the tube between the plate and the cathode, and to a lesser extent between the screen and the cathode. The more current that flows, the lower the plate (and screen) voltages get.