Other Amps

Spitfire clone sounds terrible

1

In an attempt to get back into amp building after a decade or so I salvaged the transformers out of an old Grommes PA and built a spitfire clone. I compared a few schematics and layouts found online and knocked it together

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I had to use an EZ81 rectifier since this PT didn't have a 5V winding. It powered up just fine. Zero background noise. So quiet I thought it wasn't working at all until I plugged a cord in. Running new JJ EL84's and 12AX7's

It sounds, bad. Brittle, harsh and thin. No headroom at all. Even though the EL84s are running at 16.4 watts (double checked the bias last night) the volume is lower than my 5F2-A through the same speaker cab. All my voltages correspond (within reason) to the schematics. Swapping the tubes changes nothing. All overdrive seems to be from the preamp since the sound doesn't change from full volume to dialing down the master.

Any tips for a guy who is a bit rusty in the troubleshooting department?

2

The Spitfire is quite a bright sounding amp. Was the transformer donor amp sounding ok before you built this? The P/T looks quite small compared to Matchless. I wonder if it can handle the current requirements?

Leaky cathode caps? Too high a value on the preamp cathode resistors?

Sorry - Can't really help much. Good luck! FWIW I never really thought much of the Spitfire I built with West's trannies (supposedly surplus Matchless).

3

Something very basic is wrong. I've built a few Spitfire clones and they always come out nice. I typically just buy a Hammond AO-35 for $100 on Ebay when I build mine. They have the perfect tube socket arrangement. All you have to do is re-populate the board, and add simple stuff like pots, a fuse holder, jacks and power cord.

Hard to say without looking at your build wiring. Maybe it's the output transformer? Try reversing the primary leads to the power tubes. Is it possible your OT is fried?

What are the voltage readings on the plates of the preamp tubes. It could be that they are under biased.

Make sure you have grid reference resistors on all those preamp tubes going to ground and that all your jacks are grounded.

If you could post gut picks and maybe sound clips I could suggest more.

4

Innards pics are a few weeks old and before it was completed. It has been apart and back together a few times since then so some parts are cleaner and some are messier. I will try and take some current ones today.

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I only plugged in the PA briefly to check voltages. It was a $5 swap meet find

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It had bad filters so I scabbed in an electrolytic to quell the hum while I checked voltages

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It made noise, I didn't give it a real test. I knew I wouldn't be using anything but the iron so I didn't attempt to fix it before tearing it apart.

5

Make sure the values of all resistors are correct. One that's in the thousands that shouldn't be or vise versa can be the culprit. Caps of the wrong value as well.

I've heard of people using headphones to follow the sound through the signal chain to find the source of the distortion but I've never done it myself. Basically you use simple headphones with a 390 ohm 1/2 watt resistor in series. You simply clip it in at various places along the signal chain. Play guitar into the amp and test the sound along various test points listening for distortion.

6

While it won't cause the problems you mention, I would suggest that maybe you should have flipped the board over before populating it with parts. Having the leads crossing the board so much will only add to the hum and noise.

7

Measure all of your preamp voltages, and you'll likely find the culprit. Then poke at everything with a chopstick. The headphones trick works, but you want to use a coupling cap, like a Mallory 150, to block high voltage from reaching the headphones. The value isn't important. A .1 or .047 or .022 work fine.

http://www.diystompboxes.co...

9

Finally got a few moments last night to troubleshoot this.

Last year I found this awesome Ohmite display organizer at a swap meet. Full of original resistors. Guy said it came out of his dads repair shop in the 50's.

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I thought nothing of using these NOS carbon comps in the new build. I didn't test them all and should have. Some drifted up in value a few percent. Some read 50% higher than their markings and the 1.2k in the phase inverter path read over 100k.

Little devils indeed. Lesson learned.

It's loud, it has a lot of headroom and it is very bright.

10

Vintage isn't always better. Hard to believe, tho, that they could've drifted from spec by that much. Always double check!


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