Diodes in Bassman rectifier

[robotman]

Hey guys I am running into some problems on my new build of an AB165 Bassman. The main problem is massive hum if I hook the output transformer red lead in front of the choke per schematic. If I hook it to the power tube power i can play the amp but get a delay when hitting a massive chord. I figure this is just sag because of where I hooked it in the circuit. I am using a Hammond 290EX power transformer that is supposed to be 320vac on each leg but is actually 330vac. I have a "Fat Stack" Mercury Magnetics output transformer. I am using 1N5404 diodes for the rectifier circuit (I think they are 3 amp 500 v). It isn't really a bridge rectifier but rather has 3 diodes inline on each ac lead with all the plus ends pointing towards the first filter caps. The circuit calls for 425 vdc at the output end of this rectifier. That is with the dc output anchored to 2 - 70 mf caps in series with 220k resistors wrapped around them and anchored to ground. I am actually getting 498 vdc at this point in the circuit. I am using Massive (like 2" x 7/8) Sprague 80 mf caps instead of the 70 mf called for and 1 watt 220k resistors. This part of the circuit is in front of the standby switch in the circuit. I tweaked the values of the resistors in between the secondary filter caps to get voltages around what the circuit calls for. (ie I had to use 2 - 1K resistors in series to get the "B" voltage close) I also went to a bigger choke (5H instead of 4H) to see if that would help with the hum but no joy. Is it just because I have the wrong type or size of diode that my voltage is so high and have so much hum?

[robotman]

I got some 1N4007 diodes and rebuilt the rectifier. The voltage is still too high at 491vdc. I checked the voltage from the rectifier without the 2 - 80 mf caps and it is 319 vdc. As soon as I tie the rectifier to the 2 - 80 mf caps the voltage goes back up. I tried a single 80 mf cap instead of the 2 inline and had no apparant change.

[MWJB]

I wouldn't get too carried away with the voltages the circuit "calls for", I'd expect any vintage Fender to read 10-15% high on the B+ when compared to the schem.

The 1N4007 or even IN4508 are 1000V PIV and a better choice than 500V PIV diodes. 1N4007 are fine in your amp.

The caps aren't the problem the rectifier will only produce 1.4 times the AC supplied with the filter caps there. 2x100uf (50uf total) are the values typically used here.

"If I hook it to the power tube power i can play the amp but get a delay when hitting a massive chord." Where exactly are you connecting the power tubes & the OT centre tap (red wire). The red wire goes to upstream side of the choke/downstream side of the standby. Ony the primaries (red, brn) wires should connect to the tubes.

Do you have tubes installed, drawing a reasonable load (30-35mA per 6L6) and you are measuring voltages with the standby in play mode? If your tubes are not drawing decent current this will push your voltage up.

What are your heater voltages?

As ever a pic would be useful.

[robotman]

I am checking voltages with all tubes in and in "play" mode i.e. standby on. If I hook the red lead from the OT to the upstream side (right on the standby switch with the choke lead)of the choke I get huge hum. I tried reversing the leads on the output jack but that made no difference in the hum. When I say I can play it if I hook the red lead to tube power I mean the downstream side of the choke or the "B" power. I get "normal" tube amp hum with the red lead connected to the B power but also the attendant sag/delay thing I mentioned. I have the tubes biased at minus 48.5 volts. I have not checked the current on the cathode side yet. The 1.4 x rectifier output voltage would give me 448 vdc which would work I suppose. You are saying that the 1.4 is the max I can get when using the 2 caps in series? Would reducing the rating of the caps in that position lower the voltage gain? I'll take some pictures. This is actually a Musicman 65 head chassis. I bought it unworking and tried to fix it but failed at that so I decided to make a Bassman head. The filter caps are mounted on the outside of the chassis ala normal Fender while the rectifier is mounted on the inside of the chassis at the PT end. The hum I am getting when attaching the OT to the upstream side of the choke is massive 60 cycle hum. You know I just thought of something. The speaker output jack is not grounded to the chassis. It is isolated by fiber washers I guess so the speaker ext jack could be in series with the main jack. I'll run out and try grounding the jack...

[robotman]

DUH Its all in the grounding

DUH What a nube. Yes, of course, that was the problem. I hadn't seen the isloated output jacks before and remember thinking it was a good idea. Of course, I promptly forgot about it as I continued to build the amp... With the red OT lead back on the standby switch and the neg side of the speaker jack tied to ground, there is only the usual hum from a tube amp. Since I have the tubes biased kinda hot I would expect it to hum. Now I just have to tweak everything. My voltages went to 472vdc at the downstream side of the choke/ first filter cap position (B+). The 6L6s can handle up to 500V so thats okay and besides there are the 2 - 470 ohm resitors in between the tubes and power. Now, I think I hate the power section as built, which is by schematic. It seems a little too bland/clean Have you ever "blended" 2 different Bassman schematics? I am thinking the more conventional AA864 power section with the AB165 pre-amp. I would probably have to play with that coupling cap to get it right, correct? I would also like to add a master volume. That would go after the last pre-amp 12ax7 and before the 12at7 right? Take the pin 6 output from the 12ax7 through a .047 uf cap and 220 k resistor and run it to one end of the pot . Then run from the center of the MV pot through a coupling cap to pin 2/1m resistor junction for the 12at7? The other end of the pot ties to the 22k /.1 uf / ground ref. junction on the 12AT7 resistor array on pins 3,8 and 7. I also see a 120 pf cap from the center of the pot to the 12ax7 side on the schematics that have master volumes. Is this cap to control the highs in the signal chain?

[MWJB]

A solid strate rectifier will produce 1.4x the AC applied from the PT...it'll do this whether you have a 20uf cap or a 120uf filter cap...there may be slight variations depending on cap value & how they're wired, but not as much variation as you get from rebiasing the tubes for instance.

-48.5v is not a bias measurement, it does not tell us how much current is passing through the tubes, you need to measure plate current in mA. Fit 1ohm resistors between power tube pin 8 & ground & measure the mV - if you get 35mV then that equates to 35mA.

472v does not seem unreasonable, measure the AC voltage that feeds the rectifier from the PT what do you get?

I'm not a fan of the tolex bassmans, I don't know why you would specifically build a model without having an idea of what it sounded like.

You're posts ae a bit tricky to follow, if you broke them down into paragraphs it would be easier.

[robotman]

After getting the speaker jack grounded I worked on the finer points of the build.

I made sure all of the filiment wiring was pulled up off of the chassis and at 90 degrees to just about anything that crossed it. Added some heavier wire for grounds and power runs and just generally neatened things up. I did see a post on one of the forum pages that talked about not making the wiring all parallel and perfect so that is what I did. I shortened some runs and ran them more directly. I used shielded runs for the long runs to the volume and treble pots on the bass input section because the pre-amp tube is all the way at the end of the chassis. Remember, this is a Musicman 65 chassis so I had to put all of the 12ax7s at the far end of the chassis. It is really a very quiet amp now. This amp has no more noise in it than my Fender 75 head.

I have 462 vdc on the OT red lead (MWJB you were exactly correct. 330 vac on each PT leg times 1.4 = 462), 420 vdc on the B+, 391 vdc on the "C", and 311 vdc on the "D"(had to use different resistor values to tune in the B,C,D voltages). Measured with all tubes in, half volume and standby on.

I biased the 5881s I am using to 35.5 ma at the cathode. At half volume with amp warmed up. I biased each tube seperately ( like the schematic). I adjusted the bias circuit primary resistor until I got about -48 volts before the trimmer pot. I ran that to one tubes 220k resistor and then used the trimmer pot output to make the current match on the second power tube.

The amp is pretty tight. I played it at half volume on the amp and just brought my guitar volume up to get into overdrive. It is freaking loud when you get to where it breaks up! It has a screaming lead tone at reasonable volumes though.

I tried it on my 4 x 12 and it was ripping cool. My 2 x 12 cab is a better match though, as you can keep the guitar turned up a little more without blowing your ears out. I did notice that turning the amp up all the way and the guitar up half way got some pretty gritty distortion. Unfortunately, I couldn't be in the same room with it!! I had to break out a long guitar chord so I could stand outside the shop...

Thanks to MWJB for the encouragement.

To answer a sortof question from MWJB. Why would I build an amp when I didn't know what it was going to sound like? For me it is simple. I have never heard any Bassman of any model. I don't know anyone who has one either.

I read reviews or comments in amp reviews refering to the Bassman sound as if it is a standard by which all are compared. I read that the Marshall amps of yore were basically a Bassman copy. I have had a Hotrod Deluxe, a Blues Deville, a Princeton Reverb II, a Marshall 1987XL head, a Mesa Dual retctifier head, a Reverend Kingsnake head and a slew of EL84 combos and heads. I had never bought a Bassman because it was gonna cost too much. (I ended up spending more than it would have cost to buy one ) It was only a single channel, no reverb etc. I have been building 6V6 amps of late ( Champ, Super Champ, Angela Super Single Ended, PRII power and tone section) and decided to build a 6L6 amp. I chose the Bassman becuase of all the hype.

I researched "best Bassman" on Google and found several opinions as to which model was the best. I chose the AB165 because of the following.

The AB165 has a unique design in that the second pre-amp tube has the 470k resistor feedback loop. This is, according to what I read, supposed to give the signal a more compressed sound. The power stage is a weird one too with 220k resistors looped back from the OT leads to the output side of the phase inverter. It also has a solid state rectifier. I prefer the solidstate rectifiers over the tube rectifier ("sag"or brown sound) type of amps so I decided to build an AB165.

Before I tweaked the voltages and straightened up the wiring I wasn't sure I liked the amp. After getting it tuned in it is really cool. It has a clarity and note definition that I have not heard from a Fender. It is super sensitive to pick attack and my finger vibrato never worked so good. I am sure it is not a tone for everyone though as it is a more "modern" sound than the other Fenders I have had. It doesn't have the automatic overdrive that my Princeton Reverb II did but it has a much more massive sound, fuller I guess. I am sure this is due to the 5881s (also 40-50 watts to the PRIIs 18)

Now, to be objective, this amp has a MM OT that cost $$$ and the filter caps I put in it (big Sprague Atoms) cost $60 for the 6 filter caps. I put paper in oil coupling caps in that cost so much each that I am too embarassed to say how much. All the resistors are oversized as are the pots. I am just saying that it probably does not sound like your run of the mill Bassman. LOL I STILL don't know what a " real" Bassman sounds like. I guess I am not upset because it is a great sounding amp on its own merit.

[MWJB]

I'm glad that you're a lot happier with the sound of the amp. For reference, the Tweed 4x10" 5F6A bassman is usually the one referred to as the standard by which amps are judged, this is the amp that Marshall copied for the JTM45.