how to blackface a twin ?

[hebaton]

Is there somewhere a description of what needs to be done to "blackface" a silver face twin ? (in this case an AA270 Dual showman reverb)

Thank you!

[pdf64]

Are you sure it's a good idea to do so, eg will you be able to check that the amp is unconditionally stable following the changes? The amp will have a little more gain, but will be more prone to bias shift, potentially blocking distortion.

[hebaton]

Not looking for distortion, or early OD, but perhaps just a bit warmer voice...
In any case, I will not do the work, I'll have a pro I know do it. Just trying to make things simple for him. If I could provide some guide or list of things to do. He is a competent technician, but not a guitar amp specialist...

[pdf64]

A little louder and a fraction brighter, might be the expected outcome, all else being equal.

[mojotom]

I like Silverface mods, well not all of them but the Silverface PI is nice and well suited to a 12AT7 tube.

I did rebuild a lot of them and my advise would be to remove caps to ground on power tubes and wire a regular bias pot then change the polarized capacitors (filter, bypass on cathodes and bias) and go from there until the amp is relatively low noise and balanced.

Everybody seems to Blackface those amps where they need good tubes (especially output stage and PI) and a fresh set of caps (and proper bias).

[Lynxtrap]

If you are looking for a ”warmer” tone there are other mods you could try that might be more productive than blackfacing.
Trying a different set of speakers is an obvious one, makes more difference than blackfacing IMO.

If you want to blackface it, your tech can look at an AB763 schematic to see what changes to do. Adjustable bias can be added to the bias balance with an additional trimpot so that you have both functions.

[pdf64]

Yes, the main parts CBS cheaped out on are the speakers.

[Smitty]

I have found that the primary contributor to 'cold' sounding fender amps is the bias point. If biased 'cold' (lower idle current) the amp is a little punchier but lacks warmth. Tubes also last longer when biased cold. As a side note the thing that kills tubes is moving the amp when the tubes are warm. Let them cool before moving the amp.

Fender changed the bias circuit to a bias-balance schema. In this configuration the bias pot balances the idle current between tubes but does not make the idle current for the entire output section go up or down. The function of setting idle current for the output section is handled by the 3.3K resistor from the bias board to the tap on the bias balance resistor.

You have three mod options:
1. Replace the 3.3k resistor with another resistor that will increase the idle current (good)
a. try a 4.7k to start and see if that gets you close to 65% plate dissipation at idle
b. try not to exceed 70% of plate dissipation at idle. Try 3.9K if you do...
c. this is by far the simplest mod. I call it one-resistor magic. IMO, it transforms the amp.
2. Mod the bias schema to AB763 (better)
3. Remove the 3.3k resistor from the circuit and replace it with a 10K potentiometer wired as a rheostat (best)
a. for this mod I prefer the CTS 201XR103B mounted to the bias board.
b. set bias to taste
c. this is essentially making mod 1 adjustable

You could also blackface the phase inverter but many among us will agree that the AA270 phase inverter changes represent an improvement in balance and clean drive.

[Ten Over]

The function of setting idle current for the output section is handled by the 3.3K resistor from the bias board to the tap on the bias balance resistor.

The bias voltage is set by the 1k, 3.3k, and the 15k resistors. Of the three, the 3.3k resistor is rarely changed when modding the bias circuit. Changing the 1k resistor results in a rather potent change in the bias voltage which is probably why the 15k resistor seems to be the one most often changed.

[Ten Over]

You have three mod options:
1. Replace the 3.3k resistor with another resistor that will increase the idle current (good)
a. try a 4.7k to start and see if that gets you close to 65% plate dissipation at idle
b. try not to exceed 70% of plate dissipation at idle. Try 3.9K if you do...
c. this is by far the simplest mod. I call it one-resistor magic. IMO, it transforms the amp.
2. Mod the bias schema to AB763 (better)
3. Remove the 3.3k resistor from the circuit and replace it with a 10K potentiometer wired as a rheostat (best)
a. for this mod I prefer the CTS 201XR103B mounted to the bias board.
b. set bias to taste
c. this is essentially making mod 1 adjustable

Since this thread is about blackfacing a Twin, mod #2 is the only viable option.

You don't want to replace the 3.3k resistor with a 10k pot because a wiper failure will result in the loss of bias voltage for the power tubes.

[Smitty]

if you wire the pot as rheostat (tie two legs together) it would go to 10K not open. You wouldn't lose bias voltage but you would increase idle current..

I recommended a CTS 201X. That's a good trimmer. It's unlikely to fail. It's not like it's performing wah service and will wear out or that there is enough current in this circuit to fry the pot.

The problem with reducing the 15K resistor is you are loading the bias winding which is part of one of the secondary windings. It's better to unload that winding tap so your secondaries are better balanced.

The OP was looking for 'warmer'. Swapping the 3.3K for a 4.7K is fast, easy and the best bang for the buck.

That mod and soul control will go a long way to making an otherwise well functioning AA270 into a nice warm clean amp.

[Ten Over]

if you wire the pot as rheostat (tie two legs together) it would go to 10K not open. You wouldn't lose bias voltage but you would increase idle current..

A rheostat is a two-terminal device, so when you said "wired as a rheostat", it was a very reasonable assumption that you would only be using the wiper and one end of the track.

A 10k resistance in place of the 3.3k resistance would idle a bogey 6L6GC at 100% plate dissipation. Most Twin owners would not be very amused by this, especially with current tube prices.

[Ten Over]

I recommended a CTS 201X. That's a good trimmer. It's unlikely to fail. It's not like it's performing wah service and will wear out or that there is enough current in this circuit to fry the pot.

Why tempt fate, especially when it is completely unnecessary? There are plenty of other options that default to colder bias when the wiper fails. For instance:

Twin Bias Mods 1A.JPG

[Ten Over]

The problem with reducing the 15K resistor is you are loading the bias winding which is part of one of the secondary windings. It's better to unload that winding tap so your secondaries are better balanced.

Let's get things in perspective here. The bias circuit is drawing something like 3mA. The plates can draw hundreds of mA.

[Roe]

the primary contributor to 'cold' sounding fender amps is the bias point. If biased 'cold' (lower idle current) the amp is a little punchier but lacks warmth.... the AA270 phase inverter changes represent an improvement in balance and clean drive.

yes, often the bias was around 55%. decreasing NFB may also help, but this depends on the tubes and amp (e.g. nos 7581a sound stiffer than most 6l6s and can benefit from less negative feedback). Ken Fischer even used el34s to improve the tone of twins.

Agreed on the silverface phase splitter, although I prefer a 470R, not a 270R, cathode resistor