Finally found a Marantz 8B!

This amp probably would work well with high efficiency horns like Klipsch, true?

 

Physical size. You'll never get film caps of that value anywhere near that size.

 

35 WPC is pretty loud with high efficiency speakers.

I can crank my ST-70 (35 WPC) pretty nicely on 84dB speakers, although the bass is handled by a separate solid state powered sub. It isn't as loud as I'd like for some kinds of music, but that's probably better for my hearing anyway!

 

Ok, cool. Good to know.

 

For practical reasons. Polypropylene caps of that capacity are huge.
Also, since the Marantz 8B's sonics in stock form are beyond reproach, why try to improve on it?

The Nichicon caps by the way, are of excellent quality. The ones that I use can be found in current Audio Research products.

 

Exactly!

 

I dont mean in general, I meant in the picture... they were already in there, why take them out?

 

They were installed as bypass caps to the main electrolytics, barely soldered in. Very shoddy workmanship. An electrical hazard too. They had to go.

In a classic piece like this, it totally destroys its character.

 

Ah, I see now.

 

Congratulations on the beautiful amp.

 

Hello!

I can't wait to read up on how this project turns out. If it is not to your liking Jeff, I can be over in 20 minutes to pick it up ;>). Peace. Robert

 

Good question.

The ST70 transformers debuted a new and patented (2,815,408, easily available as a pdf from http://www.freepatentsonline.com/2815408.pdf

which provided perfect AC symmetry at the expense of the same copper length in each half-primary, since one is in the inside and one at the outside of the winding. One turn inside is much shorter than one turn outside.

How big a problem does that present? Debatable.

The ST-70 OPT probably IS the best part of the amplifier though. Using those opt's one can build a fairly decent amp, but using a better transformer one can build one with much better distortion figures at a given level of NFB.

Ed Dell did an article on a homebrew power amp called "The Brute" in Stereophile (!) in '65 or so. He used the ST70 transformers for no discernible reason since at that time Peerless 20/20 and UTC Linear Standards as well as British Partridges were still stock shelf items.

The rest of the ST70 was less good. The PT was undersized from a distortion standpoint so they turned the quiescent current up, making it more undersized. And the circuit just was awful.

I hate to sound contrary but if there is one thing I DON"T like about the Marantz 8B it's the biasing setup. I would not modify a vintage or reissue Marantz, but if I were building a clone, I'd put cathode resistors in and bring out ground and the resistor drop to a set of pin jacks and put trimpots in for adjusting and get rid of all that switch, meter and complication. Today a Fluke DMM is cheaper than a set of tubes.

Provided the biasing range and heater supply are adequate, you should be able to use any output tube from a 6L6GC to an EL34, KT66, KT77, 6550 or KT88 in such an amplifier. (Only a 7027 would need a small wiring change). IIRC B+ is about 450-480 on these units and the plate load in the 4000 ohm range-the Marantz expert here will correct me if i'm wrong.

There is a specific procedure to compensate an 8B for its particular transformer and I have no idea what it is. My guess again is that they kept it obscure deliberately.

 

You will have to get rid of the Biaset resistor and put in separate cathode resistors of a rational value and separate bias pots. You can rewire the octal socket for convenient test points.

 

I went down the road of ST70 modification - new input board, new power supply capacitors (ala SDS), tube rolling, triode switching, regulated front ends, etc etc and ultimately found the Dynaco ST70 to be a mediocre amplifier. No matter what I did there remained a strange 'springiness' (for lack of a better term) to the sound. Almost like a tube compressor or something like that - and this was efficient 93dB speakers. The much maligned Dynaco Mark IIIs (rebuilt with new input boards and power supplies) ultimately proved to be a better amplifier.

One of the better sounding modifications I did to a '70 was replace the EL34s with plate cap EL38s. The EL38 requires a little less bias voltage then the stock '70 can give, so they were only biased at ~20ma each - suddenly the amplifier got BIG and DyNaMiC sounding with plenty of power reserve. I didn't take any measurements as this was just a temporary setup - but I imagine this improvement was from a higher B+ (lower current), less strain on the power transformer, and maybe the OTs worked better at lower current - dunno.

 

The stock PT is good for one channel or the other but not both. The Mark II/III had adequate power supplies.

To bring up a ST70 to the limits of what it can do you need a bigger power transformer or an external power supply.

The PT sits horizontally on the chassis and so what they do is wind a taller stack replacement on the same lams. Several have been offered over the years. The best had a primary for a bridge rectifier and no rectifier winding as well as a laddered primary for all world voltages. You'd think Hammond would have one as a stock part but they don't.

With this new power transformer, a proper bias supply (it's actually best to use a small second transformer off the heater winding) and some more reasonable front end and driver circuit, really, a ST70 is not too bad of an amplifier at all. The problem is you are paying a lot of money for a chassis, which is undersized and a pair of OLD and failure prone OPTs in the host ST70 today.

The EL38 is a pretty good tube, so are a lot of plate cap types including the 807 and 1625.

For the price of a ST70 you can buy a pair of new, better OPTs and build a chassis that has more room so you can point to point wire it all up.