AC 30 Rebuild with questionable voltages

[Mike_J]

I did a rebuild on a friends AC30 clone.  The original builder had grounds throughout the circuit board so I decided to rebuild using the Hoffman layout.  It had the LarMar PPIMV in it which seems to work well so I decided to leave it in.
The amp sounds decent and is relatively quiet with little hum and hiss but I am not sure about the voltages I am measuring.  With an AC line voltage of 120V I am getting 294 VAC on the red PT wires going to the rectifier tube.  The plates on the power tubes measure 247 VDC and the cathode 7.6 VDC.  The calculated plate dissipation per tube is 12.9 watts.
The reason for my concern is the plate voltages seem pretty low considering the PT voltage going into the circuit.  Can anyone tell me if I should be concerned?  I don’t  want to give the amp back until I know it is right.
Thanks
Mike

[sluckey]

With an AC line voltage of 120V I am getting 294 VAC on the red PT wires going to the rectifier tube.  The plates on the power tubes measure 247 VDC

I don't think that's right. What is the B+ at the first filter cap? Did you take any voltage readings before the rebuild?

[Mike_J]

With an AC line voltage of 120V I am getting 294 VAC on the red PT wires going to the rectifier tube.  The plates on the power tubes measure 247 VDC

I don't think that's right. What is the B+ at the first filter cap? Did you take any voltage readings before the rebuild?

Thank you for replying Sluckey.  The B+ at the first filter cap is 351 VDC.


I didn't take any voltages before the rebuild as the tubes were fried and I didn't want to risk damage or additional damage to the OT.


Thanks
Mike

[Mike_J]

Additional voltages.  Cap after the choke 261 VDC.  Plate voltages on preamp tubes: 1a = 108 VDC, 1b = 103 VDC, 2 = 223 VDC, 3a = 180 VDC, 3b = 181 VDC.  Cathode voltages on the preamp tubes:  1a = 1.0 VDC, 1b = .9 VDC, 2a = 1.1 VDC, 2b = 132 VDC, 3 = 48 VDC.

[sluckey]

The plates on the power tubes measure 247 VDC
The B+ at the first filter cap is 351 VDC.
Cap after the choke 261 VDC.

Those are some big voltage drops! You're losing 104V through the OT primary. And you're losing 90V through the choke. You may already have some damaged iron.

With power off and B+ drained, measure the resistance of the choke. Then measure the resistance of the OT primary... CT to each plate winding. You can do these three measurements in circuit.

[Mike_J]

The plates on the power tubes measure 247 VDC
The B+ at the first filter cap is 351 VDC.
Cap after the choke 261 VDC.

Those are some big voltage drops! You're losing 104V through the OT primary. And you're losing 90V through the choke. You may already have some damaged iron.

With power off and B+ drained, measure the resistance of the choke. Then measure the resistance of the OT primary... CT to each plate winding. You can do these three measurements in circuit.

The choke resistance is 525 ohms.  The OT primaries from CT are 91 and 105 ohms.


Thanks
Mike

[sluckey]

We need to see your schematic.

[shooter]

fwiw, If you leave your meter clipped to the 1st filter cap, VDC, does the voltage slowly drop over time, say, 5-10 minutes?

[Mike_J]

We need to see your schematic.

The layout is based from the Hoffman AC30 board and instructions with the exception that the LarMar PPIMV replaces the pair of 220K resistors feeding the grids of the power tubes.


I have a schematic that I will use the yellow highlighter method on tomorrow.  Once I have verified that the amp matches the schematic I will post it.


Thanks
Mike 

[Mike_J]

fwiw, If you leave your meter clipped to the 1st filter cap, VDC, does the voltage slowly drop over time, say, 5-10 minutes?

Can I do this test with the amp at idle or do I need to have signal going to it?


Thanks
Mike

[shooter]

amp at idle

with power OFF, hook up your meter(gator clips work well), VERIFY it'd VDC, red to +, blk on - of cap, then power up and just idling is fine. 2 amps back, my B+ was lowish, would drop over time, the 2nd day the problem cap got HOT. (was a new cap)

[Mike_J]

We need to see your schematic.

I went through the amp with the attached Hoffman AC30 schematic and everything looks alright.  As I mentioned before the only difference between the Hoffman schematic and the amp as-built is the Lar-Mar PPIMV which I penciled into the schematic.


Did the ohm measurements for the OT and choke seem okay to you?


Thanks
Mike

[Mike_J]

amp at idle

with power OFF, hook up your meter(gator clips work well), VERIFY it'd VDC, red to +, blk on - of cap, then power up and just idling is fine. 2 amps back, my B+ was lowish, would drop over time, the 2nd day the problem cap got HOT. (was a new cap)

I placed a meter across the cap tied to the rectifier tube and after five minutes it maintained it's voltage at 355 VDC.  At the same time I clipped another meter to the second cap (after the choke).  It started at 330 VDC but the voltage gradually went down to 315 VDC after five minutes.  Do you think this means the second cap is bad?  Both of these caps are new F&T 47 uF @ 500 volts.


Thanks
Mike

[shooter]

Do you think this means the second cap is bad

Not yet, pull all the 12ax tubes and repeat the test

[Mike_J]

Do you think this means the second cap is bad

Not yet, pull all the 12ax tubes and repeat the test

I pulled the preamp tubes and repeated the test.  Results after five minutes were B+: 358 to 360 VDC. Second cap: 316 to 307.


Thanks
Mike

[shooter]

Pull the PA tubes, repeat, but I suspect it will be the same.  I'd probably look for part/wiring errors and heat, if you're loosing 100+ volts, something should be hot.  swap regulator tube or try SS as a test,

[sluckey]

Here are your numbers...

power tube plates = 247v
power tube cathodes = 7.6v
calculated plate diss/tube = 12.9W (I only get 9.1W)
B+ at first cap = 351 (this should be same point as OT center tap)
Ot resistance CT to plate leads is 91Ω and 105Ω


Hoffman schematic shows 50Ω cathode resistor shared by all 4 tubes. You measured 7.6v across that resistor, so the current through that resistor calculates to 0.152A. That current is split between four tubes. Assuming the split is equal means each tube gets .038A. Let's ignore screen current and assume .038A flows into the plate. Voltage across the tube is 247 - 7.9 = 239.4. Plate dissipation is 239.4 x .038 = 9.1W.

Now here's a big one that doesn't add up... Two tube plates connect to a side of the OT. So 2 x .038A = .076A flows thru two tubes and also flows thru one side of the OT (Ignoring screen current again). You measured the OT resistance at 91Ω and 105Ω. So, calculated voltage drop across each side of the OT would be 91 x .076 = 6.92v and 105 x .076 = 7.98v. But you measured 351v at the OT ct and 247v on plates which would mean you have 104v dropped across half the OT. It can't be both ways!

I suspect either your measured values are wrong of the PT and/or OT are damaged. You did say the output tubes were fried. I'm not quite sure what fried means but I suspect they were run way too hot until they were damaged. Running the tubes way too hot can also damage the OT and PT.

How hot does the PT and OT get? Do you know that the PT is sized properly for this amp? Did your friend play this amp running maxxed out? Do you have another PT and/or OT that you could sub? Show us some pics.

[shooter]

 

the tubes were fried

I knew I shouldn't have taken that Evelen Wood course

[Mike_J]

B+ at first cap = 351 (this should be same point as OT center tap)

Wired the OT center tap to the second cap.  Is this enough to cause the voltage problem?  Guess I will rewire it and see.  Looked at the schematic quite a few times and missed that connection.  Working on getting photos.


Thanks
Mike

[sluckey]

Wired the OT center tap to the second cap.  Is this enough to cause the voltage problem?

That's not fair! 

You said it was wired just like Hoffman's schematic except for the MV. Yes, that could be the reason for the low voltage problems. Also, that choke is likely not big enough to carry the current required by the whole amp. Move the OT to the first filter cap and then you get a free do-over.   Let us know what the new voltages are.

[Mike_J]

Hoffman schematic shows 50Ω cathode resistor shared by all 4 tubes. You measured 7.6v across that resistor, so the current through that resistor calculates to 0.152A. That current is split between four tubes. Assuming the split is equal means each tube gets .038A. Let's ignore screen current and assume .038A flows into the plate. Voltage across the tube is 247 - 7.9 = 239.4. Plate dissipation is 239.4 x .038 = 9.1W.

Using your method of calculating plate dissipation I have come up with 16.8 watts after moving the OT CT wire to the spot where it was supposed to be. Results in a plate voltage of 334 VDC and cathode voltage of 10.3 VDC.  I am using a 50 ohm power resistor.  Wattage of 16.8 is too high from what I have read.  It is my understanding that it is best to have a screen voltage of 300 VDC or less when using EL84s.  After the screen voltage is in line a wattage rating of between 12 to 14 watts is suggested.  Apparently the way to accomplish the lower wattage is to increase the power resistor. Somewhere between 60 and 70 ohms I am guessing.  Does this make sense?


Thanks
Mike

[shooter]

best to have a screen voltage of 300 VDC or less

You mean plate voltage?  I like mine close to 300 for cathode biased (self biased)  In that case you would use a bigger cathode R, which will give you a bigger V drop at the cathode, minus the plate V should get the plate and screen lower.

[sluckey]

Yes, that all makes sense. And your numbers look better now too. AC-30 have a reputation for running very hot. An EL84 idling at 16W will have a very short life. I'd put a 100Ω resistor under the cathodes and see what the new dissipation is. Then just juggle it up or down until you have a dissipation number you are comfortable with. I'd be comfy with 9 watts per tube.

[Mike_J]

Wired the OT center tap to the second cap.  Is this enough to cause the voltage problem?

That's not fair! 

You said it was wired just like Hoffman's schematic except for the MV.

I looked at it a few times and just missed that connection for some reason.  Thank you for asking the question about having the OT CT tied to the B+. Probably would have taken me months to find.


I have found the preamp tube voltages for the 1961 vibrato circuit amp and the 1962 top boost add.  They are VIA: Plate = 170, Cathode =1.64, V1B: Plate = 170, Cathode = ?, V2A:  Plate = 180, Cathode = 1.65V,  V2B: Plate = 290V, Cathode = either 150 or 160V, V3A&B:  Plate = 230.  Have you or anyone else found better preamp voltages?


Thanks
Mike