Fender Bassman 50 with Master Volume

[xtian]

OK, great. Got parts ordered.

Later, I will want to mod the Bass channel to take advantage of the unused triode stage, and voice the channel for more preamp distortion. Is this the JTM45 idea?

[rdjones]

Later, I will want to mod the Bass channel to take advantage of the unused triode stage, and voice the channel for more preamp distortion. Is this the JTM45 idea?

The JTM was based on a much earlier version of Bassman than you have there.
It used a cathode follower like the 5F6's and 6G6's.
No wasted triode sections in those amps.
The JTM didn't contain any new 'ideas', it was simply an adaptation of the 5F6 circuit.
The only difference being for the tubes that were substituted.

Have we determined which circuit yours is most similar to, in the absence of a proper 50watt MV schemo ?
I know the 70watt UL was a complete re-work with only the preamps being the same.
Can you determine the value of the NFB resistor and where it connects to the PI ?
The 70W UL went back to the traditional 820/100 divider feeding the longtail cathode.
All the previous late '60s - early '70s circuits have a 47k feeding the PI grid.

rd

[xtian]

My amp is the 1974 Bassman 50, schematic below, except that there is an added master volume, and a missing 16uF/27K pair at the end of the power filter chain.

I have ordered F&T caps to update the power section, and new JJ tubes. I plan to convert the hum balance to bias adjust, move the normal channel closer to AA864 spec, and see how it sounds.

The bass channel will probably get a makeover like the attached Hasse mod.

[xtian]

Parts are in. I replaced filter caps and changed Normal channel to AA165 spec. Turned it on without tubes, and looks good so far. (Looks so much nicer with modern, smaller components.) Too late to put tubes in tonight; will look closely at it in the morning before continuing.

QUESTION: I want to install PPIMV master. In the AA165 schem, it looks like I'd be replacing the 220K resistors with the dual-ganged pot (circled in red in the attached image). But my Bassman 50 doesn't look like that. Will I be replacing the 100K resistors with the dual-ganged pot, as circled?

[thereminator]

It's bias circuit, don't touch it and don't change anything or you're gonna kill power tubes.

How to properly install post inverter MV:

http://taweber.powweb.com/store/mv_schem.jpg

[topbrent]

Here is a good visual on how to hook up the PPIMV.

Parts are in. I replaced filter caps and changed Normal channel to AA165 spec. Turned it on without tubes, and looks good so far. (Looks so much nicer with modern, smaller components.) Too late to put tubes in tonight; will look closely at it in the morning before continuing.

QUESTION: I want to install PPIMV master. In the AA165 schem, it looks like I'd be replacing the 220K resistors with the dual-ganged pot (circled in red in the attached image). But my Bassman 50 doesn't look like that. Will I be replacing the 100K resistors with the dual-ganged pot, as circled?

[xtian]

http://taweber.powweb.com/store/mv_schem.jpg

Both of these schematics show coupling caps on both sides of the MV, which is weird.

Most PPIMV designs suggest I'm going to REPLACE a pair of existing resistors with the pot. But this one you've shown looks like I'm just ADDING the pots.

Then there's the component order. Most designs show, from L to R: PI, coupling caps, bias resistors, grid leak resistors, power tubes. My Bassman has PI, bias resistors, coupling caps, bias adjust, grid leak, power tubes.

Still confused. Please reference my Bassman schem, attached.

[xtian]

"After" photos below. Kicks ass! Installed new set of JJ tubes. New power tube sockets, Normal channel to blackface spec, roughly, Bass channel cleaned up.

Normal channel gets a little crunchy when dimed, playing my LP with P90s. Nice, loud and punchy. Good tonal variation with the Treble and Bass controls.

The Bass channel is just MEAN! I just pulled the crap components and left the basics, didn't bother adding the unused triode stage yet. With the Deep switch engaged, it's got a lot of bite.

Oddly, there is some hum in the Bass channel with the Volume from 3-8, then there's a perfect place with no hum at all at 8.7, then more hum when dimed. What's that about? The pot is clean and smooth sounding, so I don't think that's an issue...maybe? Maybe an issue of how I tied the two channels together? My method looks like AA864 schem, each channel ends with 220K resistors, join together, followed by 500pF cap, into PI.

Anyway, at the 8.7 sweet spot, there's only the white noise whoosh of 400 horses ready to kick you in the teeth! Loud!

[xtian]

Here's where my channels are mixed together. Kosher?

[thereminator]

http://taweber.powweb.com/store/mv_schem.jpg

Both of these schematics show coupling caps on both sides of the MV, which is weird.

It's not weird. You're simply don't understand how tube amp works. Caps on both sides are separators for DC. Without them bias of power tubes will "floating" with MV adjusting.

I'm going to REPLACE a pair of existing resistors with the pot. But this one you've shown looks like I'm just ADDING the pots.

Don't do it. It will kill power tubes. These resistors are working in fixed bias circuit.

Scheme on the left in my link wil be great for your amp.

[martin manning]

http://taweber.powweb.com/store/mv_schem.jpg

Both of these schematics show coupling caps on both sides of the MV, which is weird.

It's not weird. You're simply don't understand how tube amp works. Caps on both sides are separators for DC. Without them bias of power tubes will "floating" with MV adjusting.

I'm going to REPLACE a pair of existing resistors with the pot. But this one you've shown looks like I'm just ADDING the pots.

Don't do it. It will kill power tubes. These resistors are working in fixed bias circuit. Scheme on the left in my link wil be great for your amp.

Theriminator, replacing the bias feed resistors with a dual pot and no additional coupling cap like Topbrent posted will produce a more transparent MV. This is the Ken Fisher "Type 2" in slightly refined form, known as Lar-Mar. No danger to power tubes whatsoever.

Xtian, I think you could replace the two bias feed resistors as shown in your schematic and leave the bias balance circuit as-is. You'd have to connect the "bottom" of the dual pots to either side of the balance pot separately, instead of tying them together and connecting to the bias supply at one point. If you want adjustable bias rather than the balance control, rewire it to the circuit on the left in your post showing both Bassman bias circuits. The dual pot for the MV is chosen to match the bias feed resistor value, which is 100k in your case. 220k's are more common, and it would probably be fine if you used a 250k pot and 2.2M safety resistors.

[xtian]

I gave up on trying to rid the Bass channel of it's annoying hum, and redid the entire circuit to AA864 spec. One notable exception was the addition of an additional filter stage at the end of the chain, like the AA165.

Now, my noise problem is worse! A fairly loud hum and hiss is pervasive. Both channels are working OK, that is, if you plug in and play, it works OK, and is VERY LOUD. But the noise floor is not acceptable.

My voltages are high (Fender recommended values in parens):
wall voltage: 121 vac
PT HT: 337 vac (305)
OT primary: 435 vdc (420)

This is after adjusting the variable bias to the recommended -44vdc.

Ideas for troubleshooting?

[Cliff Schecht]

Did you lift the grounded side of the heaters and add a faux centertap? Sorry if this has been mentioned (prolly) but it's a pretty common mistake.

[xtian]

Did you lift the grounded side of the heaters and add a faux centertap? Sorry if this has been mentioned (prolly) but it's a pretty common mistake.

My Bassman has the heater balance pot (photo below). And the red/yellow wire from the PT is the HT center tap, I believe. I think the balance pot serves the same function as the faux center tap, right? The two outside lugs of the balance pot read about 22ohms each.

[Firestorm]

You've got the tone control leads for the normal channel kind of wrapped under and around all the B+ leads coming from the doghouse. You do not want to do that. The tone leads should be as far away from those noisy leads as possible.

I also see that you've got higher than spec plate voltages, but you've set your negative bias voltage at spec. That might hum. You need to adjust the bias for the actual conditions, not to the schematic.