Converting AA371 Basssman to best sounding circuit for guitar- which one ?

[Chevy]

I just got a mid-70's silverface Bassman 50 with an AA371 circuit. Been reading up on the best-sounding circuit to convert it to for guitar players... Would that be the AA864? There seems to be a concensus on making the output stage AA864, but a bunch of different opinions on how to do the preamp. (This has probably been around a million times already... thanks for your patience.) I don't think I'll rely on the amp for break-up, but will likely just want a great clean sound and use pedals for overdrive.

One thing I did notice about the schematics.... the AA864 preamp design won't allow jumpering of the 2 channels due to the added stage on the bass channel only; I think I'd like to be abe to jumper the 2 channels for more flexibility.

Any input you have would be appreciated !

[Justin Thomas]

What's wrong with the sound as it is? I haven't met a Bassman I haven't liked just how it is when in good running order. Sure, tiny tweaks are okay, but on the whole I think they're nice amps, if you just take them for what they are - a great 2x6L6 Fender with nothing fancy. And they all came with a "Normal" channel anyway!

I'm partial to my AB165. The only thing changed is that someone removed the local negative feedback on the power tubes before I got it, and I added a bias adjust along with the balance. Someone had added a switch for the regular negative feedback loop, but I took it out because it was basically a noise switch. My Bassman is fat, thick, punchy, and warm, and I believe is the first model to allow channel-jumpering. I also had a 74 Bassman 50 w. a Master Volume, and once restored to stock, it was fat, thick, punchy, and wam. I have read about modding just the preamps to AA864 and leaving everything from the third gain stage past alone, as a best-of-both-worlds.

Give it a chance to let you know what it can do before you dig into a nice old Fender! You might like it as it is. Granted, if it's already been trashed and rebuilt, that's different! If there is something wrong with yhe sound, let the great folks here know, and they can guide you from there.

Justin

[Jazz P Bass]

Here is some food for thought.
http://www.blueguitar.org/new/schem/...4_vs_aa165.pdf

[Justin Thomas]

Thanks Jazz,

That's the one! Chevy, give that a decent read.

Justin

[Chevy]

Here is some food for thought.
http://www.blueguitar.org/new/schem/...4_vs_aa165.pdf

Perfect !!! That's what I was looking for.... thanks much.

[eschertron]

Necro-thread, I know, but I had to quote Justin's comment above.

I haven't met a Bassman I haven't liked just how it is when in good running order.

That's how I feel today, hopefully into the future as well. I won a 1970-ish Bassman-Amp on Eek!bay and it arrived in the mail yesterday. After checking it for structural soundness I fired it up and played around with it. Yup, sounds like a Fender. Opened it up to check the construction, and verified it as schem AA371. Almost a Bassman 50, with just the cap values in the bias circuit to set them apart (as far as I can see from the schems, linked below). Almost all the amp looks original, with repairs done conservatively. The power cord was replaced with a 3-prong cable, the death cap snipped out. The original 1W screen stoppers have been replaced with 470R 5W cement resistors, maybe after one failed? One of the 6L6 grid resistors has been replaced with a modern CF resistor.
The leads running to the grids of the normal and bass channel tubes are shielded wire. Is this stock? I suspect not, but don't know. Besides what's mentioned, nothing else in the amp looks to be changed. I'm sure the oil-in-paper Mallory PSU caps are due for maintenance. There's a "bit" of buzz in the signal, but it's swamped by single-coil buzz when I'm not holding the guitar just right

I'm wondering if there's any consensus on preferring the more typical NFB injection point at the base of the LTP (like in the AA165) versus this amp's virtual earth mixer? Have I lost anything but the opportunity for an easy presence control?

http://aa371
http://Bassman-50

[eschertron]

A technical question. There are 2000p caps from the grid to the cathode pins on the power tubes (see schem above). The caps land on pin 8 and then pin 8 goes to ground. If I were to break the ground connection to pin 8, and insert a 1R current-sense resistor, will there be a problem with the caps (to the grids) in series with the 1R? I'm assuming not, but thought I 'd throw it out there.

[Tom Phillips]

...If I were to break the ground connection to pin 8, and insert a 1R current-sense resistor, will there be a problem with the caps (to the grids) in series with the 1R? I'm assuming not, but thought I 'd throw it out there.

I say "no problem" with that.
Tom

[Justin Thomas]

Those caps were for parasitic suppression due to sketchy lead dress. I've been able to snip them out of all my varied Bassmans with no issues... just in the interest of losing unneeded parts.

Also, be aware if futzing with the negative feedback on these, the speaker leads might be swapped already, too... I know they are on the AB165, but haven't checked all the other schema lately.

Justin

[eschertron]

Those caps were for parasitic suppression due to sketchy lead dress. I've been able to snip them out of all my varied Bassmans with no issues... just in the interest of losing unneeded parts.

That grid-cathode cap is also on my SF champ (76) so I've encountered it before. I applaud the CBS designers for wanting to make the amps as service-free as they are, but yeah, it's not often the bean counters insist on ADDING parts. I haven't decided if I want to remove them just to see what happens.

Also, be aware if futzing with the negative feedback on these, the speaker leads might be swapped already, too... I know they are on the AB165, but haven't checked all the other schema lately.

I see that the AB165 has the same virtual-earth mixer as the AA371. You've seen no reason to redesign it?

[Justin Thomas]

I think with the caps, some oscillated, many didn't, so let's make sure none do. Easier to tack the part in EVERY amp than take the few that do oscillate back under warranty or stop the assembly line to add them in later...

As far as redisigning my amp, nope, no reason. I love the way it sounds as it is. It starts to crunch on 4 and just gets better from there. I don't know how item supposed to work, I've heard it called a 'pseudo-compressor," but I just like it the way it is. And while I'm sure Fender was trying to make a cleaner amp for bass, the fact that this "happy accident" stayed in place for 15+ years (AB165, AC568, AA270, AA371, Non-MV Bassman 50, MV Bassman 50, Bassman 70) says that SOMEbody liked it...

I kludged an extra 12AX7 into my Bassman 100 just so I could make the preamp match my AB165. And Earth cloned the AB165 for their EarthBass 1000 (I think that's what it's called?)

Justin

[eschertron]

Yes, Fender did have the sense to keep what worked for the most part. The normal channel on bot AB165 and AA371 are identical, the power amps match (except for some changes to the bias circuit), and the rest is close. The global NFB is slightly different - haven't looked that closely at it yet, though - and the local feedback around the recovery has less gain reduction on the AB165 than mine. That would be an easy place to see about changing how much I need to push before clipping the 6L6s.

[darkbluemurder]

Instead of the 2200 pf caps to ground 220 pf would have sufficed to cure any instability from the global negative feedback.

[bob p]

To paraphrase Will Rogers, "I never met a Bassman I didn't like."

Even the much maligned Bassman Ten is a fantastic amp if you pay attention to what makes sense in a guitar amp and what doesn't. The problem with the B10 was that it was truly designed to be a bass amp, having things like frequency response shaping caps in parallel with the plate loads, so some changes need to be made to make it into a true purpose-built guitar amp, which isn't really the case with the original Bassmen. They are great guitar amps straight out of the box. Take an old SF Bassman, plug it into some EVM speakers and crank it to 7 and your Strat is immediately in the SRV zone.

[bob p]

That grid-cathode cap is also on my SF champ (76) so I've encountered it before. I applaud the CBS designers for wanting to make the amps as service-free as they are, but yeah, it's not often the bean counters insist on ADDING parts. I haven't decided if I want to remove them just to see what happens.

I wouldn't remove them. There seems to be a lot of internet conventional wisdom that says that all pentode input sluggers need to be removed, and I don't agree with that. I know you already know this, but since this thread is undoubtedly going to be read by a lot of other people later on, it's probably worth reiterating.

A lot of the internet gurus will say that the pentode sluggers need to go away as a fundamental step in Blackfacing a SF Fender. The rationale behind this thinking is that if Leo didn't put them there, they don't belong there. While Leo was a true visionary, he was a furniture store owner, not a double-E. He built his amps by licensing other peoples' designs, succeeding through serendipity, and then hiring smart people to help him. In all likelihood the changes that were made by CBS were made by a team of properly trained EE who knew what they were doing.

The trend that Leo had been pursuing with the SF amps was to deliver more clean power in the same sized package. CBS continued in that direction when they took over the company. Like Leo before them, CBS had been using a double-edged sword called Negative Feedback with bandwidth limiting to increase the clean power of the output stage. They were acutely aware of the role that that transformer design (and it's manufacturing cost) played in the stability of an amp that had a high degree of NFB, and they used generally accepted engineering principles (GAEP) to maximize the amps' margin of stability. They used the textbook method of slugging the poles in the amp.

Output Transformers are complex beasts that often function in non-ideal fashions; flux density effects bandwidth, they have leakage inductance and leakage capacitance, and modelling their performance is tricky. At high frequencies they have a second order attenuation characteristic and they impart a phase shift into the signal. As more NFB is applied more phase shifted signal voltage is feed back to the feedback loop's input and the possibility of oscillation arises. We know that's why the quality of the OT can play a significant role in amplifier stability.

In a tube amp output stage the dominant pole of the NFB loop is the OT. Because the OT is sort of a sealed black box (that designed with black magic ) it's not something that we can deal with directly by changing it internally, which means that we have to look elsewhere to control oscillation.

The next dominant pole in the power tube / OT section of the circuit is the input capacitance of the pentodes. By slugging the pentode inputs with additional capacitance, one can force the pentode inputs to become the dominant pole in the NFB loop. Manufacturers typically choose a C value that is low enough to provide significant rolloff at a frequency that isn't in the range of human hearing, and certainly isn't in the range that's reproducible by a guitar speaker. In other words, those pentode input slugs are the primary effector of NFB stabilization in the amp, and they're working at frequencies that are inaudible. With those sluggers in place the closed loop bandwidth of the amp continues to extend beyond the range of human hearing and the speakers' ability to reproduce signal, which means that there's no detriment in tone.

In other words, I can't think of any good reason to remove the pentode input sluggers, and many good reasons to leave them alone.

With that said, I have to sheepishly admit that in my youth I used to remove them, convinced by the self-professed amp guru book writers of the pre-internet era that doing so would provide the amp with a more 'airy' sound. But the reality was that I was hearing a placebo effect, where I had expectations and wanted to hear an intended result. When I swept the amp for frequency response I proved that what I thought I was hearing was nothing more than placebo effect. Now I don't remove pentode sluggers. In fact, I put them in on amps that don't have them, especially on amps that are going to be subjected to intentionally distorted signals with gratuitous HF content.

Sorry for the long windedness, but having been gone for a while I felt like I needed to make a contribution and make up for lost time.