AO-10 crucial carbon composition resistors, EQ-curve

[Dik van der Noot]

I am thinking of letting my AO-10 be rebuild. One tech proposed exchanging all electrolytes,capacitors,resistors with new parts , because everything is interrelated , and one does not want new problems popping up repeatedly after restoration.. However , what I gathered from guitaramp builders , is that there are a few crucial resistors in tube-amps where it is worth checking , whether one maybe wants to stick with the old carbon composition resistors . Which has to do with more musical sounding distortions etcetc.
My question is : where are these resistors located in the AO-10 preamp schematic? One could then do a swap during final stages of a rebuild , and judge by ear which one is preferable in those crucial locations , as a last step in the rebuild.

Second question: does anyone know how much the preamp pre-equalizes the generators signal before it goes to the Leslie. Is there a diagram or measurement of this? My guess would be a gradually increasing boost starting at 200 hz extended to 3.5 kHz where the boost is at maximum. I wonder which shapes the eq-curve most , the preamp or the Leslie.
How much of an influence are the tubes on the EQ-curve? I get the impression from techs that new tubes might lack in mids compared to NOS tubes.


NOTE: english is not my native language, sorry for possible odd phrasings.

[David Anderson]

It strikes me that replacing all the passive parts in an AO-10 would be extremely expensive unless your tech is working for nothing.

A properly qualified tube amp tech would already know how to identify the resistors and capacitors that need to be replaced. In general, they are the resistors that set the operating points of the tubes, plus any others that are too far out of tolerance. Electrolytic capacitors and paper dielectric capacitors should be replaced. The ceramic capacitors in AO-10s tend to be very stable.

The guitar amp world is the source of much misinformation and mythology.

In an AO-10, old-stock tubes are pretty much your only choice except for 6SN7s. No one makes 6SJ7s or 6SC7s anymore, so there's no point in worrying about new production tubes.

I'm not getting into cork-sniffing discussions about tone. If the AO-10 is working as designed, it will sound like it's supposed to.

[Dik van der Noot]

Thank you David, your insights are very much appreciated by me , an electronics nitwit. There is a highly respected ampguy in my neighbourhood, but he is not inclined to do the job. I have to search some more.
I really dig the term "cork-sniffing"-discussion , I will incorporate that in my expanding english vocabulary (although I am a beerdrinker).
There is a lot of mythology, talking about sound is a great source for misunderstanding , and a great way to avoid simply studying 6 hours a day to improve ones skills on an instrument.
But I love the whiff of a Hammond-preamp in the morning.

[Bodie]

Replacing resistors and capacitors is easy and inexpensive, I've done that myself many times. However some of the HT (high voltage: 240-400VDC) multi capacitors (many caps in one can, as they did back then) are very costly as no one makes them anymore. They usually get replaced with a number of readily available parts, any valve/tube repair guy will have a stock. If you can live with physically it not being the same, that's fine. Sadly I've had to replace a number of transformers too, costly but without which, no amplifer.
Keep all the old parts, sometimes when things are not quite right having the old parts for reference is always reassuring.

[Baseman norm]

CE Manufacturing makes replacement capacitor cans to original specs with original Mallory equipment.

[David Anderson]

Replacing resistors and capacitors is easy and inexpensive,

Ah, the DIY Fallacy. It's only inexpensive if you only consider parts and price your own labor (including the time taken to source the proper parts) at zero dollars per hour.

For example, I'd estimate I easily put $2,500 worth of work into restoring my Farfisa Compact Duo (which was in poor shape).

(And then, of course, people don't count the labor of Organ Forum members as free consultants when they rebuild something themselves and find it doesn't work when they're done. Let's face it. We get a lot of those. :-))

I don't consider AO-10s easy to work on when you're desoldering and replacing components with their leads wrapped around terminals that will break off if you're not careful with them -- especially in the front end where you have six plate/screen resistors that share one B+ terminal.

[Dik van der Noot]

Ah, the DIY Fallacy. It's only inexpensive if you only consider parts and price your own labor (including the time taken to source the proper parts) at zero dollars per hour.

For example, I'd estimate I easily put $2,500 worth of work into restoring my Farfisa Compact Duo (which was in poor shape).

(And then, of course, people don't count the labor of Organ Forum members as free consultants when they rebuild something themselves and find it doesn't work when they're done. Let's face it. We get a lot of those. :-))

I don't consider AO-10s easy to work on when you're desoldering and replacing components with their leads wrapped around terminals that will break off if you're not careful with them -- especially in the front end where you have six plate/screen resistors that share one B+ terminal.

I am not inclined to do it myself , I want to pay somebody who has worked on hundreds of preamps. That kind of experience and the measuring tools of a tech seem important to me when , besides exchanging crucial parts , you want sort of a checking / blueprint of the parts that are not exchanged , maybe encountering an anomalie. And of course, not damaging anything in the process. Even the mere technique of soldering , to do it properly , reliable and with care, is certain
y of value to me.

[Bodie]

Wow, amazing. Speaking as a Pro myself, replacing R and Cs is easy and inexpensive, for those people paying a tech will be at the mercy of their fees.

I've restored a number Farfisa Compact Duos (horrible job) very sore fingers, all the transistors on all the oscillator cards, ouch! Then troubleshooting every one, added some time. I did a very good tutorial website on this very subject many years ago! Again the parts costs, was less than £20, but thay do take quite a few hours. Asking $2500 or even the equivalent in £ is excessive in my opinion, it would be a crime for anyone to use my tutorial to fix FCDs and then seek to charge $2500. Anyone can put a price on their own time, it's what a clinet will pay.

I offer my select clients special rates, they keep coming back for more with yet more interesting things for me to work on.

Just a thought, Dik, if you are in NL, check out where Thjis van Leer gets his kit worked on.

[Hoaxel]

Or contact Brian Davenport, it ist not too far from nl

[David Anderson]

Again the parts costs, was less than £20, but thay do take quite a few hours. Asking $2500 or even the equivalent in £ is excessive in my opinion,

I think you may need all the facts before forming an opinion.

My Compact Duo came in completely non-working condition without a F/AR power supply, so I had to build a complete replica of the F/AR from scratch, including the reverb circuit, so that included finding an enclosure, building the circuit by hand on vector board, sourcing the appropriate power transformers, and finding an overall layout that worked to minimize hum pickup between the power transformers and reverb tank. That was a lot more than 20 pounds worth of parts. Just finding a 7-pin Amphenol socket may have cost that.

I replaced all the electrolytic capacitors in my Duo, including the 72 or 73 1uF caps at the output of each tone. I think replacing those 73 caps alone took 4 hours. Mine is also the Duo Mk1 for which no complete schematic exists, which added to the time. I added impedance buffer circuits to the audio output PCB. Some of the generator cards had problems on them that were not obvious and were hard to track down. Then there were the cosmetic and hardware issues. Some of the missing hardware was BSW, unavailable in the U.S., so I had to tap out the fittings to fit U.S. screw sizes. It all adds up.

I wouldn't ever likely get $2,500 for it, but I definitely put that amount of work into it, going by hours.

There's a website on a Novachord restoration, and I would wager that guy put $10,000 worth of work into restoring it.