67-68 Bandmaster

[plaush]

A Conundrum of Sorts

Well, what an interesting journey this has been. I replaced all of the filter caps as well as the bias cap (100uf). There was a loud hum at first but that went away. Things started to settle down so I plugged in a guitar. The guitar came through fine. Some of the pots are a little scratchy and I intend to get back to them later. They were not terribly scartchy. Just a little scratchy.

There remained a slight hum which went away with the volume knob so my guess is preamp circuitry as the culprit. I figures "ehh... so far so good, as I go through and learn this thing I will eventually replace whatever is causing the hum.

Now for the interesting part (depending on how you look at it I suppose), The tubes are glowing with a bright color orange. Too hot for my taste and based on my limited experience with the few tube amps I've owned, way too hot for normal operation. So now, it's time for me to check the bias. I go to check the bias and not many of my readings a making sense to me. I'm using a couple 6L6's from an old Plush Amp that was given to me. The tubes do not appear in that bad of shape. The original tubes were RCA's with some noticable arcing on the sides of the tubes (inside the tubes) so I tossed them.

The 470 ohm resistor and diode next to the bias filter cap check out to be ok.
I set my meter to DCA 200 mA range, put one lead at center tap (What I believe to be B+) of the OPT and the the other lead to pin 3 of one of the power tubes. The amperage gets higher and higher and higher. I had to change the range on my meter to get a final reading of .350 AMPS?? OMG! I think to myself. What have I done? Now remember, the guitar was coming out of the speaker just fine. I must be missing something. There are two tubes so I am reading half of the OPT's output, but still 175mA's?
I change my meter to the 1000v range and check the voltage from pin 3 of a power tube to ground. 450-460 Volts DC.

No matter where I place my meter and however much I tweak the bias pot I see no change on the large OPT current between B+ and pin 3 of a power tube.

I disconnected the wires and checked the resistance of the bias pot and the resistor going from the 3rd lug to ground. Everything ohmed out fine. Same with the two resistors on the bottom of the power tube sockets. I get the sense that I am missing something here.

Finally, I decided to try a couple different tubes so I pop in a couple realistic 6L6's out of the afformentioned Plush amp and observe...

Their warming up... I'm watching the milliamps... their climbing... 150...160... one of the tubes is starting to glow... then... the milliamps started going down... down further ... that's when I realized that the fuse had blown

Anybody, anything?

[plaush]

Well, what an interesting journey this has been. I replaced all of the filter caps as well as the bias cap (100uf). There was a loud hum at first but that went away. Things started to settle down so I plugged in a guitar. The guitar came through fine. Some of the pots are a little scratchy and I intend to get back to them later. They were not terribly scartchy. Just a little scratchy.

There remained a slight hum which went away with the volume knob so my guess is preamp circuitry as the culprit. I figures "ehh... so far so good, as I go through and learn this thing I will eventually replace whatever is causing the hum.

Now for the interesting part (depending on how you look at it I suppose), The tubes are glowing with a bright color orange. Too hot for my taste and based on my limited experience with the few tube amps I've owned, way too hot for normal operation. So now, it's time for me to check the bias. I go to check the bias and not many of my readings a making sense to me. I'm using a couple 6L6's from an old Plush Amp that was given to me. The tubes do not appear in that bad of shape. The original tubes were RCA's with some noticable arcing on the sides of the tubes (inside the tubes) so I tossed them.

The 470 ohm resistor and diode next to the bias filter cap check out to be ok.
I set my meter to DCA 200 mA range, put one lead at center tap (What I believe to be B+) of the OPT and the the other lead to pin 3 of one of the power tubes. The amperage gets higher and higher and higher. I had to change the range on my meter to get a final reading of .350 AMPS?? OMG! I think to myself. What have I done? Now remember, the guitar was coming out of the speaker just fine. I must be missing something. There are two tubes so I am reading half of the OPT's output, but still 175mA's?
I change my meter to the 1000v range and check the voltage from pin 3 of a power tube to ground. 450-460 Volts DC.

No matter where I place my meter and however much I tweak the bias pot I see no change on the large OPT current between B+ and pin 3 of a power tube.

I disconnected the wires and checked the resistance of the bias pot and the resistor going from the 3rd lug to ground. Everything ohmed out fine. Same with the two resistors on the bottom of the power tube sockets. I get the sense that I am missing something here.

Finally, I decided to try a couple different tubes so I pop in a couple realistic 6L6's out of the afformentioned Plush amp and observe...

Their warming up... I'm watching the milliamps... their climbing... 150...160... one of the tubes is starting to glow... then... the milliamps started going down... down further ... that's when I realized that the fuse had blown

Anybody, anything?

Correction: the voltage was actually 358-360VDC

[Bob M.]

First power-up...

After extensive work on any amp the first thing I usually check is the bias on the power tubes. I would recommend setting the bias pot to obtain a reading around -50Vdc ~ -52Vdc at pin 5 of each power tube (you're using 6L6s, right?). This will keep the tubes idling in a range that won't damage them or anything else while you check out the amp voltages: B+, screen, PI, preamp and heater. Once you're convinced everything is running as it should (normally) then return to the bias circuit and re-set the bias mAs for each power tube for their optimal setting, taking into account the plate voltage measured. If you check for the presence of a bias voltage, even before you take the amp off standby, you'll be doing yourself a favor. Bias is one of the most critical voltages in an amp, set it first...

Bob M.

[plaush]

Once I get Another Fuse

Thanks Bob. Right now I have 6L6's in the amp. I am planning on using 6V6's once I get everything up and runnning. I am assuming that the bias setting is measured between pin 5 and ground. I remember taking that reading but I am not sure how I got onto the readings. Something wasn't make sense at the time I think. Right now my head is swimming with all of this. I hope to put in some 1 ohm resistors for future biasing.

[plaush]

The Latest

Thanks for your recomendations Bob. For all of those following this post I did immediately replace the cord with a nice three prong one.

At one point nothing I did would stop the way the tubes where burning so hot. A bit discouraged but eager to learn I ventured on.

The first thing I decided to do was change the 470 ohm bias resistor and the in backwards diode tied to one end of that same resistor. VIOLA! I immediately got a good reading between pin 5 and ground. My readings went from 58 down to 38 while adjsuting the bias pot. I've got some new tubes to put in it but while I continue to work on the amp I am going leave the old tubes in.

I think the next thing I am going to do is clean and retension all of the tube sockets. Check and clean the pots.

Any thoughts on how hot to run the 6V6's ?

[Sweetfinger]

That amp doesn't have an adjustable bias. By that time Fender had switched to a bias balance pot. It wouldn't matter how the thing was turned if the bias voltage was incorrectly set, or missing altogether. If the tubes really were drawing over 100 milliamps of current, there's something wrong. Is there bias voltage present at all?

[plaush]

Negative Bias Now

That amp doesn't have an adjustable bias. By that time Fender had switched to a bias balance pot. It wouldn't matter how the thing was turned if the bias voltage was incorrectly set, or missing altogether. If the tubes really were drawing over 100 milliamps of current, there's something wrong. Is there bias voltage present at all?

After replacing the resistor and diode I got between -58 and -38 VDC. I did replace the capacitor with a 100uf. I hope to get into it again soon. Once I do a few other things I am going to try a couple different approaches at checking and setting bias. The amp seems to be working fine now.

[plaush]

Back In The Saddle

Got this thing back together. Full set of new tubes. Set the bias. It sounds great so far. I am waiting for an opportunity to crank it a bit.

I played it through the original Bandmaster speaker cabinet. The speakers didn't sound that healthy. Since everything is original and the cabinet is so bulky I'm probably gonna leave it as is. Maybe sell it down the road.

[TimC]

Nice work plaush.