Technical Information

Here's a list of some of the most common technical questions we get asked and their answers. We've also included some Internet links to help out.

Power Tubes and Bias
Preamp Tubes
Tremolo and Vibrato
Cabinet Rattles and Speaker Buzz
Schematics and More Amp Info

Power Tubes and Bias

Do I need to bias my new power tubes or can I just plug them in and go?

In the best of worlds you would have a qualified tech install your tubes, check your bias and adjust for best tube performance and longevity. In a good portion of vintage Ampeg amps such as the Gemini I, the power tubes are "cathode biased" and are self biasing. It doesn't hurt to still check the current draw on these amps but unless the new tubes are way out of whack (pretty rare) you can just plug them in and go. Most other amps, Ampeg V4, Fender Twin etc. use a fixed bias system. The bias is set at the factory on these amps to run their particular type of power tube so generally speaking. you should be able to plug the right tube type in and go. But, it is common for these factory set bias supplies to drift so it is best to have it checked and set up by a good tech. Your tubes glow when they are on from the filament, the part that heats up the tube components. But an easy way to tell if your tubes are in trouble is if the plates, the gray metal on the outermost part of the tube glows a bright red. This indicates an "under biased" situation in which the tubes are literally being fried! It's easier to see in a darkened room. Bottom line we recommend having your power tubes installed by a qualified tech to save yourself any potential headaches and for best results out of your new tubes.

Are the 7027A, 7591A and 7868 power tubes used in vintage Ampeg amps available?

Thankfully there are two companies making 7027/7027A power tubes. Electro-Harmonix has been making the 7591A for a couple of years now and we've been very pleased with them. For now no one is offering the 7868 although I've been told it's in the works from Electro-Harmonix. Otherwise you will need to modify your amp with new 8 pin tube sockets. These will allow you to use other more common tubes such as the 6L6 with a few other alterations to the circuit.

Preamp Tubes

What about those weird preamp tubes in Ampegs, 12DW7, 6CG7, 6BK11 etc..?

A good portion of these tubes are in current production, 12DW7, 12BH7, 6CG7 etc.

However the Compactron type tube are not and probably will never be made again. This includes: 6BK11, 6K11, 6U10 etc. Hold onto your old ones for backup. Good news is they tend to last a long time in the circuit. There are currently new old stock available on these and there are some substitutions that can be made. Just keep in mind that these will eventually disappear.


What caused my reverb to stop working?

Here's a short list of things that can cause reverb problems.

1.      A bad reverb tank

2.      Weak driver and recovery preamp tubes, In Ampeg Type "C" systems this is usually the 6CG7 tube and a 12AX7.

3.      Bad tank cables

4.      Bad footswitch

5.      A bad reverb transformer as used in Fenders

Tremolo and Vibrato

Why doesn't my tremolo/vibrato work?

Here's a short list of things that cause tremolo/vibrato failure.

1.      Bad or weak Tremolo or Vibrato Units. Causes weak trem/vib in vintage Ampegs

2.      and can cause volume loss when trem. is turned up.

3.      Bad or weak capacitors in trem/vib circuit.

4.      Weak tubes in trem/vib circuit.

5.      Bad footswitch.


My amp has a low end hum no matter what volume I play, why?

Most likely your amp could use a "cap job". The filter capacitors in your power supply are weak and failing. You may also have a bad power tube, rectifier tube, diode, or power transformer. Remember -- on a lot of Ampegs and other vintage amps there is a hum balance control. Try adjusting that first to see if you can remove the hum. Sometimes this control goes bad too. Otherwise have a tech check these other items to effect repair.

Cabinet Rattles and Speaker Buzz

If you are experiencing extraneous noises from your amp cabinet check:

1)Tighten all baffle, latch and speaker connector nuts, screws etc..

2)Portaflex gasket failure may cause air leaks.

3) Speaker Grill cloth flap especially on 60's amps can sound just like speaker buzz. Best fix is to put felt  under the grill cloth.  May need to replace grill cloth at this point or we can make you a whole new baffle or grill.

4) Plywood voids and separation. The old plywood may have voids or delaminated over the years causing rattles and buzzes.  A cabinet specialist may be able to repair but will probably require a  tear-off and re-covering  of the amp. Very hard to fix.

5) Cabinet Gasket on Portalfex amps. Air may be leaking out of the top of your cabinet due to old gasket.  Replace with new gasket.

6) Speaker deterioration.  Pull speaker out and inspect for tears, rotting etc. Small tears may be repairable. Otherwise re-cone your original or try one of our recommended replacements.

Feeling overwhelmed? Don't want to deal with it? We offer a full line of electronic, speaker and cabinet repairs. See our Services page for more details.

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