New 5F2A, not sure voltages are good.

[spsimmons]

I just finished up a 5F2A kit from Triode. It sounds good, but not as loud or dirty as I expected. It might be that it is what it is, but some of my voltages seem off from what other people have reported. I can't find a particularly good resource for what they should be. This is my first build, so I'm a little surprised that it actually works at all. It's even quiet, as in no hum (maybe that's a bad sign).

How do these readings look? All were taken to ground, with all tubes in.

Sovtek 5Y3GT

pin 2: 423v
pin 8: 423v
pin 4: 342vac
pin 6: 342vac

JJ 6V6 S

pin 2: 3vac
pin 3: 407v
pin 4: 362v
pin 5: 0
pin 7: 3vac
pin 8: 23v

JJ ECC83 S

pin 1: 189v
pin 3: 2v
pin 4: 3vac
pin 5: 3vac
pin 6: 188v
pin 8: 2v
pin 9: 3vac

[Bruce / Mission Amps]

I didn't know they sold a 5F2A kit.

Unless SOVTEK changed something recently, the SOVTEK 5Y3GT is not a real 5Y3... it's more like a GZ30 or 5V4GA....
Try a real NOS 5Y3GT in there and your B+ rail will come down and so will the power tube's idle current come down a bit.
If you are using a 470 ohm cathode biasing resistor at 23vdc... that is about 49ma through it, of which probably 45ma-47ma is all plate current.
So your plate to cathode voltage now is 384vdc and the tube is idling at say 46ma... .046a X 384v = 17.7 watts.
Kinda high for a 6V6.

[spsimmons]

Thanks Bruce,

You're right, I built the amp from Triode's 5F1 kit. I added a tone control and separate power switch. I differs from a 5F2A in a couple of ways: I opted for the 15w OT, it's only running 1 16uf filter cap instead of 2, it also doesn't have the 25uf cap between the pre-amp tube and ground (parallel with the 1.5k resistor).

Thanks for the very clear explanation. I've seen plate voltages from the high 300's to high 400's reported in amps similar to this. Now I understand how and why that's not the whole story. I'm slowly figuring this stuff out. I will certainly order up a NOS 5Y3. Do you have an opinion about the JJ 5Y3? I might be able to find one of those around town.

Also, my understanding is that running my 6v6 out of spec will shorten it's life span. I may have to chance it til the new rectifier comes, really enjoying playing the thing.

[spsimmons]

I actually found an old Conn branded 5Y3 in a box under my work bench. I popped it in and the amp works great. I haven't pulled the chassis to check the voltages after the tube swap, but will probably do that this weekend.

I still continue to wonder about the volume, or lack thereof. Last night I took the new amp to my regular jam session. The other guitarist plays a Mexican Standard Strat through a Silvertone 1457 (amp in the case). The 1457 seems to be a very similar amp in many ways. It has a 6V6 power tube with a 12ax7 pre-amp. It does have a tremolo circuit and a different rectifier. It plays through an 8" speaker.

The rub is that his rig was a good step louder and cleaner than mine. I figured that the 10" speaker in mine would at least give me a volume edge. Instead, he had his amp at maybe 3/4 volume and could adjust on his guitar some. I had to keep my amp and guitar maxed out just to sort of hang with him. Does this indicate a problem with my amp or do I just need to build a 5E3? I would have thought that the 2 5watt amps would be a great match, so far this is not the case. Any ideas? What could cause a lack of volume?

[loudthud]

There could be an ultrasonic oscillation going on. Try disconnecting one side of the 22K resistor that goes from the speaker jack back to the 12AX7. The amp will have more gain, so to compare apples to apples you will need to turn the Volume control down some, but does it seem to play louder while still clean?

[Dave H]

I just finished up a 5F2A kit from Triode. It sounds good, but not as loud or dirty as I expected...

If it doesn't get loud and dirty it could be short of gain. You don't have a cathode resistor bypass cap on the first 12AX7 stage. Try adding one as on the 5F2-A schematic or disconnect the output stage 22k negative feedback resistor.

[spsimmons]

Thanks for the suggestion loudthud. I disconnected the resistor as you suggested and there was no discernible difference. I wasn't sure about it so I hooked up a test lead between the 22k resistor and the speaker jack so that I could listen to it, then un-clip it and listen again. No difference to my ear.

Does this suggest poor ground from the speaker jack?

I also took voltage readings again, this time with the Conn 5y3. All of the voltages were down. The plate voltage on the 6v6 (pin 3) is now 383vdc and the cathode reads 21vdc (pin 8). This means that my 6v6 is still running at 15.9 watts? If I'm reading the JJ data sheet, I think it should be at 14 watts or less.

My math: 383vdc (plate) - 21vdc (cathode) = 362vdc (plate to cathode). 21v / 470ohms = 44ma current. .044a x 362v = 15.92 watts

[spsimmons]

So, I definitely have continuity from the speaker jack to ground. Disconnecting the negative feedback makes no difference.

Two potentially interesting discoveries:

1. Plugging into input #2 produces noticeably more volume than plugging into input #1.

2. My electrolytic caps seem to be draining themselves. As soon as I turn the power off the voltages drop to single digits, within a couple of seconds.

[spsimmons]

So, I keep monkeying with this amp, trying to get it to act right. Three interesting symptoms remain.

1. Input #1 produces noticeably less volume than input #2. Shouldn't it be the other way around?

2. The electrolytics drain themselves rapidly when I turn the power off.

3. Disconnecting the negative feedback resistor produces little or no effect.

What I've done:
-Re-flowed all the solder between the inputs and V1
-Installed a NFB switch between the 22k resistor and the speaker jack. (I had it loose anyway.)
-Removed the connection between input jack #1 and my star ground.
-Installed a switch between the .0047uf cap for the tone control and ground. (This added gain, as expected.)
-Installed a second 16uf filter cap
-Installed a 22uf, 35v cap parallel to the 1.5k resistor between pin 3 of the 12ax7 and ground.
-Turned the amp on with a 250 ohm resistor clipped between B+ and ground.

[loudthud]

#1 Look in the messages tab on my profile for a link to a thread where I described how to test the input circuit with an ohm meter. It was written for a two channel amp, but you should be able to figure it out.

#2 Try the same test with the power tube removed. That should slow down the discharge.

#3 Hard to say what's causing this, square away #1 and #2 first.

[Dave H]

#3 Hard to say what's causing this, square away #1 and #2 first.

The feedback resistor could be open circuit or too high value. It would be easy to use 220k by mistake. I'd disconnect one end of it and check its value. I'd also check that it's connected to the hot not the ground side of the speaker jack.

sp, Disconnecting the feedback resistor should produce a similar gain increase (about 6dB or x2) as adding a bypass cap to the first stage cathode resistor.

[spsimmons]

Thanks so much loudthud and Dave H.

I think you guys have helped me make some progress here. My DMM (Greenlee DM-20) wasn't doing a great job of taking solid resistance measurements, but what I got was pretty interesting.

No cord

-pin 2 to gnd: 34.3k
-pin 7 to grd: 94.1k

cord in input 1

-pin 2 to gnd: 68.1k
-pin 7 to gnd: 94.1k

cord in input 2

-pin 2 to gnd: 1030k
-pin 7 to gnd: 94.3k

cord tip to ground

-input 1: 137.5k
-input 2: 578k

cord tip to pin 2

-input 1: 169.4k
-input 2: 34.4k

The filter caps do in fact drain more slowly with the 6v6 removed. They still "self-drain", but now it's minutes not seconds.

The NFB resistor measured 227k. Hmm.

So, you've helped me uncover some issues. Now I can dig in and try to figure out why things aren't as they should be.

One is, where did the 22k resistor go? Maybe between the grid of the 6v6 and ground....

[Bruce / Mission Amps]

...
The NFB resistor measured 227k. Hmm.

So, you've helped me uncover some issues. Now I can dig in and try to figure out why things aren't as they should be.

One is, where did the 22k resistor go? Maybe between the grid of the 6v6 and ground....

Oh yeah!

[spsimmons]

Big thanks to loudthud, Dave, and Bruce!

My amp woke up tonight. I had a 220k and a 22k resistor mixed up. Doh! I would have probably never figured that out on my own. When I put them in their correct positions in the circuit, the volume of the amp nearly doubled! Great clue Dave!

I also figured out my input 1 vs input 2 issue. I'm a BONEHEAD! A tweed amp chassis is traditionally laid out VERTICALLY. My amp is in a 90 degree, or horizontal chassis. By transposing the vertical layout to a horizontal layout, I flipped the whole control panel. My fuse is on the far right and the inputs are on the far left! I was reading from left to right #1, #2. In other words, the whole problem was that I mis-named the inputs. Trouble shooting per loudthud's instructions finally helped me see that dufus mistake. (I also think that I now understand what you meant by pin 2 OR pin 7. They are the corresponding pins on each HALF of the 12ax7. I would be using one OR the other for that input function.)

I still think my filter caps are draining at an unusual rate. With or without the 6v6, they drain off real quick. It's a little quicker with the tube in.

Thanks so much, guys! It's a rocker now. A big part of the fun for me is that I keep learning at every step and with every mistake. You guys are an invaluable part of that process. I even ended up with the NFB switch and Tone Knob bypass switch mods as a bonus.