1970 Pro Reverb--flabby, farting bass...

[mnexsen]

Mostly happens on the open E string; I get a loose, flabby distorted sound. Is this normal for this amp? I've replaced preamp and power tubes, filter caps, interstage coupling caps, checked bias. Would changing rectifiers tighten things up or is that too risky? The spec'd tube is a 5U4GB.

[Gibsonman63]

Have you tried a different speaker cabinet so you can hear the amp through different speakers? Sometimes it can make all the difference in the world.

[Stonepark]

Mostly happens on the open E string; I get a loose, flabby distorted sound. Is this normal for this amp? I've replaced preamp and power tubes, filter caps, interstage coupling caps, checked bias. Would changing rectifiers tighten things up or is that too risky? The spec'd tube is a 5U4GB.

I have the same amp (73) with same problem.
Also doesn't seem very loud.

Any success????

Cheers

[Bruce / Mission Amps]

I think the Pro Reverb uses a Vibrolux Reverb - Bandmaster Reverb sized OT... realistically, it is only about +35 watts, but it uses a larger power tranny sized like a Super Reverb... at closer to 45-50 watts.
Using a 50 Watt OT for 4 ohms sounds really nice!! A Twin Reverb's little brother.
Use a total of around +50uF for the first filter cap section and keep the power tubes idling at around 13-15 watts each with the smaller output transformer... 17-18 watts each if you use a 50 watt output tranny.

However, I think the best thing to do first (for personal tuning) is go through the preamp stages and reduce bass response a little.
Try 2.2uF to 4.7uF cathode bypass caps in the preamp and try using two 47nF caps in the tone control section instead of the standard 47nF with 100nF.... bass and mid range. I actually prefer the classic 22nF pair with about 250pF to 360pF treble cap side on the Vibrato Reverb section and a 22nF with 100nF and 250pF for the normal channel.
You should try replacing the 100nF phase inverter caps with 47nF too.

Also, make sure the little coupling cap that drives the phase inverter is no more then 2200pF... I used to find a lot of amps in for fix-up where some knucklehead before me bought into the G Weber BS about using a .01uF to .022uF cap in that spot.
It sucks.... I like 1500pF to 2200pF max myself.

[VMR]

Also, make sure the little coupling cap that drives the phase inverter is no more then 2200pF... I used to find a lot of amps in for fix-up where some knucklehead before me bought into the G Weber BS about using a .01uF to .022uF cap in that spot.
It sucks.... I like 1500pF to 2200pF max myself.

I totally agree with Bruce on this point. It's the most critical "tone" cap in the amp, IMO, as everything funnels through it. I have "saved" many SF Fender amps that sounded "off" by changing this cap to .001 or .002uf.