First Time Build Problems

[dwilliamsaudio]

Wow, I'm really sorry guys. Really sorry. There was a fundamental flaw that I overlooked in all my testing. A dim-bulb tester with a 200 Watt bulb. My heater voltage is actually 6.6, not 5.78. Now my B+ measures 433. That's quite high I think. I don't mind feeling a little dumb, but I'm sorry I gave you all incorrect data.

[ThermionicScott]

Aha! The dim-bulb tester was connected the whole time?

That's a relief -- I was completely out of ideas. With a 5Y3 rectifier, your B+ should come back down to normal. How is the sound now?

Enjoy that amp!

[dwilliamsaudio]

It's still really clean I think. It is 11pm and I live in an apartment, so I didn't really crank it yet. Would the lower B+ from a 5Y3 give me more gain? The cheap Jenson MOD speaker this kit came with is supposed to be really efficient. Perhaps installing my new Weber 12a125a will allow me to crank it higher.

[Dave H]

It's still really clean I think.

Your DC voltages look OK. I'd be looking for a wiring fault in the signal path. A 5E3 because of its funky back to front vol pot wiring should start to break up at about 3 on its vol pot (with the guitar vol set to 10). It should be really loud in an apartment by then. It can't do distortion at low volume. Try it with the 300mV test tone. It should be clean at low (very low) vol pot setting and really dirty at full vol (12?)

EDIT: The volume controls are interactive, for the test set the unused vol to 0 so it doesn't attenuate the signal.

[dwilliamsaudio]

This test might have been particularly telling. Although, I have no idea how to fix it. I ran a 1khz sine wave through at 300mV as you suggested. The other channel at 0. I don't have an oscilloscope, but I have to say, it was completely clean all the way up to 12. It was also unbearably loud starting at 3. But yes, I did take it up to 12. It did stop getting louder at about 5, which I think is normal.

I also want to point out, as you may have noticed, this kit does come with a negative feedback option that you apply by using a jumper. I have tried it both ways and I do not have it in the wrong position. I also checked for unintentional continuity between the jumper pins and that is fine as well.

[ThermionicScott]

This test might have been particularly telling. Although, I have no idea how to fix it. I ran a 1khz sine wave through at 300mV as you suggested. The other channel at 0. I don't have an oscilloscope, but I have to say, it was completely clean all the way up to 12. It was also unbearably loud starting at 4. But yes, I did take it up to 12.

Where did you have the tone knob set? When turned way down, there is much less total gain than when it's cranked along with the volume.

One thing you might check is that you have the right values for all your cathode bypass capacitors and that they're oriented the right way -- if they're not doing their job, you'll have a lot less gain. Just one makes a noticeable difference.

For all the talk of tweed Deluxes being "dirt machines" and having "no clean headroom", my homebrew* which is absolutely running as it should does a great job with super-loud warm cleans at home. Perhaps with a drummer pounding away, it would break up a little bit more.

- Scott

* For the sake of brevity in that sentence, I saved the full description of said homebrew for this footnote... it's a wide-panel tweed Pro circuit using Deluxe Reverb transformers, which are actually not far from the original specs for the early Pro. Some tweed Deluxe kits just include DR transformers, so when I plug in 6V6s and a 5Y3 (actually an equivalent type 80 due to the 4-pin rectifier socket, but I digress again), it behaves exactly like many clones. And is still really loud.

[Dave H]

I ran a 1khz sine wave through at 300mV as you suggested. The other channel at 0. I don't have an oscilloscope, but I have to say, it was completely clean all the way up to 12. It was also unbearably loud starting at 3. But yes, I did take it up to 12. It did stop getting louder at about 5, which I think is normal.

I bet it was loud! Can you get hold of a 8 or 10 ohm 50W resistor to use as a dummy load? It's kinder on the ears. Using the dummy load and test signal wind up the volume until you hear the the tone from the OT get 'angrier' (that's the onset of clipping) then measure the voltage across the load to calculate the output power.

If it has a 12AX7 in the first stage it should only need about 10mV peak input for full undistorted output so it should have plenty of distortion with a Tele. Yes, you are right it won't get louder after 5 because of the way the vol pot is wired.

[Dave H]

For all the talk of tweed Deluxes being "dirt machines" and having "no clean headroom", my homebrew* which is absolutely running as it should does a great job with super-loud warm cleans at home

My homebrew does 'clean' just fine. I think the "no clean headroom, dirt machine" rep. comes from the way the vol pots work. "Man, if it's this distorted on 4 what must it be like on 12?" when in reality it doesn't actually get any louder or more distorted above 4.

[eschertron]

I don't have an oscilloscope, but I have to say, it was completely clean all the way up to 12. It was also unbearably loud starting at 3.

The way our ears are wired, at unbearably loud sound pressure levels, the ears generate their own distortion which may be hard to distinguish from single- to low double- digit second harmonic distortion. A scope might show a flattening of the waveform that's not necessarily audible on a sine wave. Next time you can smash it with a guitar do that test

I have a couple champ-type amps that really grind. But that's because the single-ended power amp really gets ugly during cutoff. The push-pull amp won't exhibit that. Maybe you need to find some wildly mismatched tubes? Just kidding.

Did you get a hold of a 5Y3 rectifier to try?

[dwilliamsaudio]

I have to admit, the tone was most likely set on 0. My caps are all correct values and orientation. It didn't hurt to check again though. I suppose it's possible I overheated something at some point. I do have 2 more sets of caps for these positions because I always buy extra parts. Maybe I'll test again and install those this weekend.

[ThermionicScott]

I just had another thought: in addition to being really irritating at high volumes , 1kHz is also really high up on the guitar's range -- about the 19th fret of the high-E string or so. With a note so high, it's harder to hear distortion products than say with a 220Hz tone, so you might play around with the test frequency.

IMO, it's kind of unfortunate that guitar amps latched onto the same 1kHz test frequency as other amplifiers, when they could have picked something more representative of a guitar or bass's range, but it does make it really easy to hit the clean power target.

[dwilliamsaudio]

I might try that. I have actually been trying to buy a Weber attenuator to use as a dummy load, but their web site is down.

[dwilliamsaudio]

I have a 5Y3 on the way. I didn't get the Sovtek because I've seen people say it keeps the B+ as high as a GZ34.

[ThermionicScott]

I might try that. I have actually been trying to buy a Weber attenuator to use as a dummy load, but their web site is down.

That's a bummer -- I noticed a few days ago that I was having trouble accessing some pages, but now the whole site is down.

You might try this in the meantime: HS25 8R F ARCOL / Ohmite | Mouser Cheap and perfect for this kind of testing. If you wire a speaker and a ~50Ω/5W resistor in series, and then put that in parallel with the dummy load, you can get a much quieter audio reference for what's going on.

[danman]

I was going to ask if you had tried to lift the yellow NFB wire from the output jack. Temporarily removing the wire and disabling the NFB should give you the extra gain that you are missing.