Rennovating Bassman head...

[Bob M.]

I'm working on a Fender Bassman Head, AB165, for a friend. He's a bass player and wants a tube amp for small and possibly medium-sized venues but he has many other bass amps as well. This amp was converted in the past by another tech to the AA864 circuit but there are many mistakes and some bad technique - alot of 'flying parts' besides the about dozen and a half of badly drifted resistors. As I'm almost rebuilding the whole thing, I'd thought I try some these fast acting or fast recovery diodes for the rectifier but I don't know much about these. Anyone up for a quick diode tutorial (or point me to a good informative website) and possibly some diode recommendations that would be appropriate for the amp I'm working on.

Thanks,

Bob M.

[Justin Thomas]

Hey Bob,

No comments on the diode questions, but would you be willing to rebuild it to stock? I use mine for bass with efficient speakers, and it's more than enough for small to medium venues. Not sure what kind of music he's into but it.com s a great rock bass and guitar amp.

The only thing not stock in mine is the 220k local feedback resistors on the power tubes, and it has a Twin Reverb OT. That's how I got it...

Just some thoughts from an AB165 lover. Bonus on keeping it original: channels can be mixed, and are in phase, unlike the AA864 & AA165.

Justin

[Bob M.]

Justin,

You make some good points but I am constrained by what my friend and client wants me to do with his amp. After all, it's his amp. There are just as many people who feel that the AA864 is the definitive circuit for a BF Bassman head. I took on the work with three concepts in mind: increase power slightly, increase headroom as much as is practically possible and increase fidelity. I'll be working with various ideas with this and other Bassman circuits to further those goals. For right now, I'm thinking about trying some tests on fast recovery diodes to see if this fits into my plan. OTOH, I don't want to move the sound of this amp too far away from what this amp does right now. I'd like to be about the same thing but better; at least better for bass guitar. I'm not trying to rebuild this into flotsam/jetsam rev 2.

Bob M.

[Justin Thomas]

No problem, I just took from your original post that it was a bit "messed up." I also know the boards are different, which sometimes leads to "creative" wiring solutions...

Justin

[olddawg]

“Fast Recovery” diodes for rectifiers are used in switch mode power supplies where they are dealing with a frequency MUCH higher than 60hz.... generally in the 10kh or more range. I can’t see how a fast recovery diode would make a damn bit of difference in performance in a conventional power supply if used in place of the standard, run of the mill rectifier diodes. A slightly higher current rating might increase reliability, but how may rectifier failures do you see with modern diodes?

[g1]

My understanding was that the fast recovery helped with the diodes turn off switching noise, eliminating the need for snubber caps we sometimes see across the rectifiers.
That being said, I don't see them being adopted in modern guitar amp manufacture, so I don't think there is a critical benefit to be had.

[Gingertube]

Below is something I posted to another forum talking about diode reverse recovery spikes. This was in response to a request for an explanation.

I hope you find it useful and not just more confusing.

I have replaced standard 1Nxxx type diodes with Ultrafast Soft Recovery types in at least 8 tube amps and I have found it always makes an improvement.


Whenever you have a P and N type semiconductor material the P type has an affinity for electrons (called holes - somewhere for electrons to go) and the N type wants to give electrons up for each of them to attain their most stable states. This has to do with the properties of the doping elements.

When you bung em together to form a PN Junction in a diode (Anode and Cathode respectively) or a transistor junction electrons flow from the N to the P creating a "depletion region" around the junction - that is its depleted of "holes" and electrons. The electron flow is terminated when the electric potential created by this flow of electrons (charge) is sufficient to resist any more flow. The depletion region will have a certain width and a voltage across it.

The junction (depletion region) will therefore has an electric field across it and hence has capacitance.

This junction capacitance has a Reverse Recovery charge (Qrr). That is, before a diode can turn off this stored charge has to be removed (in technical jargon - by recombination of majority carriers). In simpler terms - Current has to flow to remove (sweep out) this charge.

Therefore as the AC voltage reverses on a rectifier diode there will be a pulse of current in the wrong direction before the diode actually switches off. This puts a noise spike on the power supply rail which has very high current rise time (di/dt). Filter capacitors are not very good at removing (shunting to ground) this noise spike because they have a small amount of inductance and inductors resist current change. Therefore you end up with a noise "SPLAT" on the power supply rail.

The differences in Silicon, Ultrafast Silicon, Schottky and Silicon Carbide Schottky of interest when using them as rectifiers is how fast they can turn off and how large this Current "SPLAT" is before turning off.

This is obviously directly related to their Reverse Recovery Charge (Qrr). Small capacitors across each diode can help to absorb this splat and for some time they were in fact required by Electromagnetic Interference (EMI) standards for all commercial products (at least in Europe - I don't know if this is still true).

So here is the useful (representative ONLY) Qrr data:
Standard Silicon Diodes Qrr approx 500 nC (nano-Coulombs)
Ultrafast Silicon Diodes Qrr can be down to 100 nC
Schottky Diodes Qrr 50 to 70 nC
Silicon Carbide Schottky Diodes Qrr <20nC

The other thing you need to know is that the noise spike associated with the current "SLAT" makes your amp sound seriously crap and that its edge speed is often fast enough to radiate into anything that looks like remotely like an arial (resistor leads, connecting wire etc). It can make the sound very "harsh" and is typically noticed mostly in the smoothness and detail of the high frequency "top end". This applies to HiFi Amps in particular (because of the extended high frequency response) but is also noticable in Guitar Amps.

That reverse recovery current "splat" will couple back through the transformer into other windings and so noise spikes can turn up on bias supplies and heater supplies too.

From that Qrr data you can see that Ultra Fast Soft Recovery diodes will have at most 1/5th of the switching noise of the standard silicon diode - coz the Qrr is 1/5th.

Cheers,
Ian

[g1]

The other thing you need to know is that the noise spike associated with the current "SLAT" makes your amp sound seriously crap

Forgive me for twisting your words around, but what this implies to me is that if I don't think my amp sounds 'seriously crap', I'd best not mess with the rectifier diodes. ?

[Gingertube]

Also be aware that the post I copied above was from a Tube HiFi Forum.

Cheers,
Ian

PS - When I do Bassman restores the first thing I do after a recap is replace the 470 Ohm 2W Carbon Film Screen Resistors on the Output Tubes. They are usually way out of tolerance resistance wise and it is not unusual to find one or more open circuit or blown up. I usually change them to 5W Wirewound and increase their value to 1K.