AM864 repro?

Unwilling to pay over $1000 on evilbay for one of these lumps I am looking at DIY-ing my own unit. Most parts required seem to be available at the moment. I would sub the input transformer with an Edcor WSM600/10K and the output transformer with an Edcor XSM10k/600.

While at it I thought it could use noval base (9-pin) tubes. Here is the possible substitutions I have come up with:
6SQ7 - 6AV6 (very good match of parameters)
6SN7 - 12BH7 (close enough to GT version)

The 6SK7 sub with EF83 does not look close enough in transconductance and current parameters. I don't know enough to decide what parameters really count in this vari-gain application. Are there other alternatives? If not I will just go with the original octal setup. The cost is about the same.

You could probably get it to work with other tubes. But since the originals are still available, and common, why not try them first and see how you likes the flava? :wink: I heard one of these for the first time this week at Soundguy's studio, and it's a cool box, unique-sounding.

Those octals are readily available, and octals are much better than mini-9 bases. Do it right.

Octal it is then.

Do you think the EDCOR would work in a pushpull output circuit?

Why wouldn't it? It's got a centertapped primary.

i've been thinking about the same thing. i was originally thinking about just using a well-balanced center tapped transformer at the output, but PRR pointed out in the 6k7 compressor thread that any imbalance in the gain reduction tubes will create more current flow during gain reduction than other factors. i don't know if the XSM series is gapped or if they are relying on the size of the iron core to avoid dc saturation.

so, i would say, figure out the max dc current consumption during GR (which might be pricey overkill; anyone have a better suggestion or practical method for determining a safe value?). contact edcor, ask them if they'll make a custom transformer to that spec. while you're at it, ask about a power transformer and 15H choke. could be one stop shopping.

the octal tubes are all readily available, but hifi tweaks have pushed the price of *sn7 tubes up a bit. still, for about $50 you should be able to get a set with matched 6sk7s if you shop around.

i'll be interested in any progress.

ed

hi
all traffos are made by Stanley transformer corp.
power transformer is 100/220v secondary 330VCT 30madc 6.3V-2A 5V- 2A.
choke 15H 300 ohms input 600ct 10kct +25db output 10k 600.
cans are not the same .
hope it can help.
audioforge

Regarding the concern about the XSM10k/600, it takes 50ma DC on the primary so we have 25ma per tap. The 6SN7 is rated at no more than 20ma anode current per triode. I doubt that the circuit was designed to push max specified current through the triodes. More likely they run at 10ma or less, considering that the power supply is rated at 30ma only.

that's 50ma DC + AC, but yeah, if the max power supply draw really is only 30ma DC, i doubt the 6SN7, even cranked, could put over 20ma AC through the primary.

the XSM series is relatively inexpensive, which makes it very appealing. but edcor are making push-pull amp output transformers, the XPP series, that are designed specifically to have b+ fed into the center tap. all that they list are plate to speaker models, but i have found that edcor are willing to make whatever you want. the XPP series is only a few dollars more than the XSMs and are built on a bigger (EI75 vs EI625) core than the XSM series. might be worth a look is all i'm saying.

ed

[quote author="edanderson"]that's 50ma DC + AC, but yeah, if the max power supply draw really is only 30ma DC, i doubt the 6SN7, even cranked, could put over 20ma AC through the primary.
ed[/quote]

Exactly! :thumb:

i've been thinking about doing one of these also. it seems simple enough. and there is a good copy of a schematic available. I've got absolutly no experience with compressor/limiters before... only mic preamps / line amps. The controls on these things are labeled 'current control' and 'threshold', i think. however when reading up on this device, it seems the 'threshold' control is not a threshold control like we think of today. I think i read somewhere it behaved more like a ratio control. with an older design like this do you think we should be very descriptive with the controls? or is simpler better...? 'input level' , 'compression' and 'output level' knobs for instance. As long as the controls make sense and allow us to operate the unit intuitively, do we really need to know exactly what they do? somebody help me out here. what does the 'current control', or whatever it's called, doing here?

joe