59 Bandmaster Too much distortion

[buickwilson]

Have a 59 Bandmaster 5E7 that sound great to about 3 or 4, but turn it up to 5 or 6 or more and it start to distort more then it should. Gets very compressed. A bandmaster should sit somewhere between a Pro and Bassman in terms of bottom end and volume, but my PRO is much louder and cleaner up to about 7. I'm thinking the Bandmaster has a power supply sag issue.

The OT (came with a Merit replacment) and Choke (came with a no name small choke) were replaced with Mercury Magentics. It has a stock PT. Found a 47uF cap in the bias ckt, so I replaced with the 100uF per the schematic. Also found a 4.7M resistor in the feedback on V2, so I replaced it with the 10M per the schematic. All other values are stock. It has the stock filter caps and coupling caps. They show no signs of blisters or bubbles, but I know... I'm playing with fire keeping the old filter caps installed.

Tried several combination of tubes. B+ runs around 410V, but bias is around 25ma to 30ma, which is a bit low for the 6L6GB's or 5881. These are NOS Sylvania, Tung Sols, GE, RCA which sound great in my Pro and Bassman. Rectifier is also NOS Ryatheon and Tung Sol 5u4GB (I don't have any 5U4GA's). I don't think Tubes are an issue (played with preamp tubes too).

I played some transients (hard notes or chords) with the Bias Rites installed and watched the B+ dip around 40 to 50V swing on the transients. My Pro swings about 20-30V on transients with a 5U4GB. Not sure how much voltage dip is normal, but guessing 40-50V is probably more then normal. I tired a SS rectifier, which shows only about a 30V dip. It sounds better, but I would rather run with a tube rectifier and closer to the original voltages.

I went back to the 5U4GB and changd the 56K resistor in the bias ckt to ground changed to 47K. This brought the bias up to 35mA and B+ now is 395V at idle. Sounds much better, but still more distortion as it goes up past 5 or so on the volume control, Still has a 30-50V B+ sag on transients...

Can filter caps look ok, but low on supplying the juice. I figure this is next on the list, and then there's not much else but the PT...

[Exothermia]

If the filter caps are original on an amp that old, they need to be replaced. Period. It should be the first thing you do when you work on a vintage amp.

-and yes, they can look fine and be totally shot. When I did my amp last year, they were original, they didn't show any signs of leakage or damage, but they were totally worthless. I replaced them with Sprague Atoms and the hum and hiss instantly disappeared.

[tremolux]

What EXO said!!.......not to mention the overall suckiness of any 5U4 type rectos IMHO. I dont like'em at all.....in anything, tho I respect the opinions of people who are ok with 5U4GBs and use them. Try this amp with "known" good, clean, tight speakers and see what the results are...after changing out the filters.

[Blue Strat]

90% of the electrically leaky caps I've replaced looked fine. You're wasting your time looking at anything else until you've replaced these.

Check preamp plate voltages too.

[jetlag]

I have a '58 bandmaster with newish filters. With a 5U4G and 120Vac on the line, you should be in the ballpark of 440 Vdc on the plates. With a GZ34 you should be in the ballpark of 450 Vdc on the plates. Both values quoted with 6L6GC type tubes idling around 60 - 65%.

[tommytomcat]

I built a 5e7 kit and had similar issues with to much distortion once I got above 3-4 on the vol knob. I did some preamp tube experimentation. I ended up liking v1=5751, V2 & V3=12AT7 and I biased the amp to where there is as much on the plates as it will do as it's wired (about 420v). The tube changes made the amp much more guitar vol ctl and pick dynamics sensitive. Bass and treble notes are more defined/3D. BTW.. I found the v1=12AY7, v2 & 3 = 12AX7 to still be too dirty for my tastes in my amp.