AB165 AA864 Hybrid

[NickC]

Finished soldering up a new build; a blend of Fender Bassman AB165 and AA864 circuits. Both channels are "normal", but one is voiced a bit differently. Both channels go through three gain stages. The Phase Inverter and Output section are mostly AA864.

I tried something different (for me anyway) with the grounding scheme. I installed an insulated brass bar as a grounding rail for the preamp, and grounded it near the input. The bias circuit has its' own ground, as do the power stage and main filter caps. I grouped the power supply filter caps with the corresponding circuit sections (preamp, phase inverter, etc).

I'll be firing it up this weekend for the first time, checking voltages without tubes, and dialing in the bias for a matched pair of 6L6GC tubes. If that goes smoothly, I get to hear it for the first time.

[xtian]

Love your buss "plank"!

Looks great, Nick, so I guess I'll have to pick at small clams! Your green Earth wire is very short--it's supposed to be the longest of the three from the wall service, so that it's the last to get disconnected if the mains wire is ripped out of the chassis.

Why did you ground your heater CT (green w/yellow) on the preamp's buss bar? That's a high current circuit and should be kept away from where the preamp sips its electrons. I'd suggest grounding it with the HT CT and reservoir caps.

Looks like an exciting build, especially since you can jumper the channels. Waiting for the report!

[sliberty]

In my 864 copy, I did some things....

1. Bass channel only
2. Added bright switch
3. Changed the deep switch - switches between 25uF and 1uF cathode cap for first triode. (I didn't have a .68uF on hand)
4. Pre phase inverter MV. Simple and very effective.
5. Non-switchable bright cap on MV.

Plenty of cleans when MV is cranked. Planets of going when the man volume is cranked.

I love this amp.

[ToneMerc]

Why did you ground your heater CT (green w/yellow) on the preamp's buss bar? That's a high current circuit and should be kept away from where the preamp sips its electrons. I'd suggest grounding it with the HT CT and reservoir caps.

I have deployed the "Larry grounding" method on several builds with good success, it terminates the filament center tap at the preamp node as well.

TM

[NickC]

Why did you ground your heater CT (green w/yellow) on the preamp's buss bar? That's a high current circuit and should be kept away from where the preamp sips its electrons. I'd suggest grounding it with the HT CT and reservoir caps.

I have deployed the "Larry grounding" method on several builds with good success, it terminates the filament center tap at the preamp node as well.

TM

Yes, the reason I put the heater CT on the buss bar is based on the "Larry Grounding" method which I've had good luck with. But the proof is in the pudding, and I'll find out whether the amp is quiet, or may require a different grounding approach.

[NickC]

Love your buss "plank"!

Looks great, Nick, so I guess I'll have to pick at small clams! Your green Earth wire is very short--it's supposed to be the longest of the three from the wall service, so that it's the last to get disconnected if the mains wire is ripped out of the chassis ..............

Thanks! I'm hoping the "bus plank" works, so I won't have to walk the plank.

You are correct about the ground lead! Ordinarily I'd have that wall AC ground lead longer. In this case I'll be mounting the chassis into a pine, blond tolex cabinet with a 4 ohm 15" Eminence guitar speaker. I'll be anchoring the 15' AC cord to the cabinet with several heavy nylon cable clamps securely screwed in. That AC cable isn't going to get pulled loose.

ToneMerc mentioned the "Larry" grounding method, which has worked well for me before. This time I attempted to marry the Larry scheme with a a modified Fender brass plate. Only I insulated my brass buss "plank" with ceramic standoffs, and the ground wire bolted to the chassis at the far corner input jack area.

I have my fingers crossed this will prove to be quiet.

[DenDanger]

Very nice looking build!
Have fun with it.

Denny

[NickC]

Love your buss "plank"!

....................

Why did you ground your heater CT (green w/yellow) on the preamp's buss bar? That's a high current circuit and should be kept away from where the preamp sips its electrons. I'd suggest grounding it with the HT CT and reservoir caps.

Looks like an exciting build, especially since you can jumper the channels. Waiting for the report!

I ran through the initial start-up today. (1) Lightbulb limiter (2) Voltage checks without tubes (3) set negative bias voltage to max (4) installed tubes and took measurements (5) installed tubes (6) set bias for 6L6GC with 464v on plates to 44.9ma. Drained caps between each step and verified there was no voltage using multimeter.

Nothing burned. No electrocutions. But I did have to reverse the secondary OT leads to the octals.

Everything works, tone controls, bright switches, as expected.

However, there was more background hiss that I like. Chop-sticked and found the "bus plank" to be unsatisfactory. Tapping yielded much rice crispy snaps, crackles and pops. So it must go. I will instead implement the "Larry Grounding", following it to the letter of that law.

Otherwise, the amp sounds good. Draining the caps again right now and trying to decide between:

(1) if there is enough time to redo the grounding scheme before Super Bowl 50 kick off.

(2) maybe I should wait to finish the amp until tomorrow. That way I'll still have an amp to work on, and won't experience any amp-building-withdrawal symptoms overnight.

Another report on the grounding situation to follow.

[pdf64]

Why did you ground your heater CT (green w/yellow) on the preamp's buss bar? That's a high current circuit and should be kept away from where the preamp sips its electrons. I'd suggest grounding it with the HT CT and reservoir caps.

In normal operation, I don't think that there will be any current carried by the heater CT, as it's just setting a reference voltage about which the balanced heater legs will, er, balance. The heater legs / green wires carry the current.

Some power tube shorts may send appreciable HT current down the heaters to 0V, in this case via the CT, so it's a good idea to ensure it can be accommodated.

Merlin's excellent document http://www.valvewizard.co.uk/Grounding.pdf indicates that trial and error can help identify the optimal 0V reference point used for the heater CT. The input 0V reference point is likely a good place to start / default to.

there was more background hiss that I like. Chop-sticked and found the "bus plank" to be unsatisfactory. Tapping yielded much rice crispy snaps, crackles and pops..

Have you scoped for ultrasonic oscillation? The above are possible symptoms of this. But bad 0V connections can cause it, so re-arranging them may solve it.

[NickC]

Update:

I redid the grounding, removing the "brass plank". I followed the "Larry" grounding scheme (except for the speaker jacks which are grounded Switchcraft on the back panel.

I replaced some carbon comp Rs with higher wattage rated metal film types, and changed some cap values.

It's running quiet, smooth, and LOUD now into a 15", 4-ohm Imminence Patriot speaker with a Weber Beam-blocker installed.