New AB673 deluxe reverb combo build

[aussietim]

Thanks guys. I think I will go under I'm using tezfel shielded for the input cables, I'm using a TAD fibre board which doesn't seem to have full in and out holes so I'm just going to scoot the wires underneath everything
I'm using a Hammond 270BEX power transformer which doesn't have a centre tap for the heater wires. Does that mean I do not ground it with the resistors at the lamp? and just run it straight through the lamp in and out?
Curious Martin what is the trimpot on the PI plate load resistors?
If anyone can post a glory shot of their preamp buss bars I would appreciate it particularly how you have earthed it to the chassis
This was my 18 watt build that I built from scratch based on a Trinity SIII but I will be attacking it and Plexifieing it as it is too clean and I have an oversupply of clean amps

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[martin manning]

I'm using a Hammond 270BEX power transformer which doesn't have a centre tap for the heater wires. Does that mean I do not ground it with the resistors at the lamp? and just run it straight through the lamp in and out?

With no center tap on the heater winding you need to add the 2x 100 ohm resistors for a ground reference.

Curious Martin what is the trimpot on the PI plate load resistors?

PI balance trimmer as seen on Dumble schematics

If anyone can post a glory shot of their preamp buss bars I would appreciate it particularly how you have earthed it to the chassis

See the marked-up layout in my DR build thread. I prefer that system to using a ground bus.

[aussietim]

Just about finished the chassis stuff, now for the eyelet board
Any constructive criticism welcome

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[robnik33]

I built a RobRob Fender Princeton Reverb delete and found these mods the most useful. Very similar topology so the results should apply

- master volume, I did PPIMV as I read somewhere that in the AB763 topology most of the distortion is from the preamp stages.

I’ll be interested in your results if you are going T2LM. I did try Type 3, didn’t like it. I’m about to do a VibroVerb build and deciding which MV to use.

- mid control, RobRob’s implementation gives scooped mids at one and gets Tweed-ish as you increase

The combo of MV and mid gives great control over distortion at any volume. Highly recommended!

- NFB switch. Found this nice but the least useful. It matters more when you are running the amp flat out as it tailors the distortion profile.

Oh, and I’m running a GA-SC64 12” in it, it is a good choice.

Nice looking build!

[aussietim]

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Slowly getting there in the end I'm doing this pretty well stock standard, hoping to finish in the next week or so
Question does the polarity of the choke matter there are 2 black wires I assume one from the outside of the coil and one from the inner is this something to consider or am I getting carried away?

[martin manning]

does the polarity of the choke matter

I recall seeing one forum post claiming that choke polarity made a difference in the sound, but I don't believe it.

[pompeiisneaks]

does the polarity of the choke matter

I recall seeing one forum post claiming that choke polarity made a difference in the sound, but I don't believe it.

Yeah I can't even imagine 'directionality' in wire. I've also heard HiFi enthusiasts buying wire that was directional and costs a fortune... if it's just a wire, wound around a core, then I also cannot see any way it's 'directional'

~Phil

[aussietim]

Ok another dumb question Im using a Hammond 290 BEX power transformer, there is a white and a black 240 volt AC input wires here in Australia the white would be active (positive) and the black neutral (negative ) is this the case with this transformer, being AC does it make any difference? I saw an American layout where the Black wire is positive and it has confused me

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[martin manning]

The polarity doesn't matter. I would follow you local color convention for the primary circuit, and perhaps that is what Hammond intended.

[aussietim]

Getting down to the business end now, will spend a bit of time rechecking the build then start up and scope
Any constructive criticism / obvious errors welcome

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[aussietim]

Ok pretty well all done but I have a problem My voltages at the output tube plates is way high at 470 volts. Also if I try raising bias higher than about - 50 volts I start getting redplating in both 6v6's I am using the correct transformers etc
Hammond 290BEX power transformer and Hammond 1760H output transformer
The only variations from standard values are I used 1 watt value resistors rather than 1/2 watt resistors. I have checked all values and are all OK

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I will post all my voltages tomorrow but in the meantime if anyone has any suggestions on how to rectify this please post.

[martin manning]

Change the rectifier to a 5U4.

[sluckey]

6V6s in that amp should be happy with about -35V on pin 5. If your tubes redplate with -50V, then something is wrong. Try another pair of 6V6s.

How are you checking the bias? I suggest installing a 1Ω resistor between pin 8 and chassis ground for each 6V6 and monitor mV at pin 8 when checking bias.

Here's my schematic with voltage readings for comparison...

http://sluckeyamps.com/tdr/tdr.pdf

[aussietim]

Thank you both
At this point I am concerned at the high plate voltage at 470 volts it is about 55 volts higher than it should be and about 45 volts higher than Sluckeys schematic
Surely if I bring the plate voltage down the bias voltage will fall into line as well
The rectifier is an 5AR4 I have not got a 5U4. but can order one if that is the solution will it overload the current draw on the power transformer?
Another aspect that is puzzling me is my B1 voltage is about 475 volts which is about 15 volts lower than it should be according to the specs I am using which is the Stewmac ones
what is the relationship between the B1 and the plate voltage? as one is low and the other is very high?
Tim

[aussietim]

I will put those resistors in tomorrow thank you Steve