EL84 mod questions

[whitelabrat]

Hi

So I did some reading and to my surprise, it would seem that I could convert my Marantz 8B to SET and/or bridge the input/output to make it a monoblock. Not that I'd ever do that but I do have an old PP EL84 amp that I would consider applying this idea to. Anyone try this? Bad idea?

The same EL84 amp also has 3 12AX7's per channel and more gain than I could ever need. Without having a schematic handy, what are the chances I could remove two of the three 12AX7's to simplify things? Can I simply remove the tubes or must I rearrange the circuit some? I assume the latter.

[gofar99]

Greetings, Possible, maybe. If you can post a copy of the schematic I'm sure some of us might have suggestions.

Good Listening
gofar99

[whitelabrat]

No such thing... yet. I've got to trace everything to put a schematic together. A lot of the guts are in the front panel. The best I've got so far is a couple photos

look for the Calbest:
http://mysite.verizon.net/oldamps/

[gofar99]

Hi Again, Looking at the photos, two things sort of jump out. One I would leave the Marantz alone. Vintage Marantz equipment is fairly good and nearly anything you might do would decrease its value. The newer amp though looks like it could be modified fairly easily. The circuit board seems to have been set up with the outputs and drivers on one side of a central dividing line. Everything else is on the opposite side. Sort of a modular arrangement. This is fairly common in equipment as a manufacturer can make several similar pieces of equipment on essentially the same boards. I would suspect that the output from the preamps (which from your post have too much gain) is by a single capacitor or jumper from one side of the divider to the other. They might also go to / thru the control panel. Cutting the circuit at that point and installing a volume control and perhaps a coupling capacitor might well be all that would be required to accomplish what you desire. If there is an existing volume control between the two parts of the circuit you could make the break there.

If some one else out there has thoughts on this I'd be curious. PS does the newer amp have any sort of label or designation. It might be possible to search the web for a circuit diagram.

Good listening
gofar99

[Eli Duttman]

The same EL84 amp also has 3 12AX7's per channel and more gain than I could ever need. Without having a schematic handy, what are the chances I could remove two of the three 12AX7's to simplify things? Can I simply remove the tubes or must I rearrange the circuit some? I assume the latter.

I'm going to guess the 3 'X7s per channel are phono section, tone control, and driver/splitter.

The "iron" looks decent and that's a good sign.

Simplification is definitely possible, especially if the effing tone control circuitry goes away. Immodest proud papa that I am, I'll suggest you use the "El Cheapo" splitter/driver and keep PP EL84 "finals". PP 6V6s would be fine too. To use PP O/P trafos with SE circuitry requires parafeed topology, with the addition of heavy and costly plate loading chokes.

Closing the openings the PCBs go in with sheet Aluminum seems reasonably easy. Of course, you could design and etch replacement PCBs.

BTW, I don't think the "Calbest' is recent. It looks like there's a Sep., 1969 inspection date stamp on the chassis.

[whitelabrat]

I most certainly won't fool around with the Marantz 8B, but it was given me to use and enjoy rather than to be a museum piece.

The Calbest is an ancient Japanese model sometimes branded as a Musicall. 1959 and rare, but surprisingly good sound when compared to a standard like the Marantz. I don't think I have any use for the phono stage in this amp, but the tone controls are important. There's also a lot of spaghetti inside that could be cleaned up.

I'm thinking it may be better to leave well enough alone and just focus on solving the weird zapping sound it makes. I'd sell it, but I'm afraid a replacement would cost 10 times as much!

[gofar99]

Greetings, Apparently I got in late on part of the stream, Zapping?? When? A few things come to mind, but details would help. I also would keep the EL84 PP configuration. They are decent tubes and can work in triode, UL or pentode mode. I prefer the UL for a compromise in power and sound quality. I'm in the process of building a new amp with them and will post how it goes at a later date (the trannies haven't come yet).

If you need tone controls (I won't go into the arguments for or against) Lighthouse Electric on the web sells a low gain tube kit for about $60US that is fairly good. I used one for a while. The biggest problem with it was it needed real clean power and the bass and treb controls had to be kept away (or shielded) from A/C sources.


Good listening
gofar99

[Ty_Bower]

Originally posted by whitelabrat
The Calbest... I don't think I have any use for the phono stage in this amp, but the tone controls are important. There's also a lot of spaghetti inside that could be cleaned up.

I had a Harmon Kardon A230 with a similar layout. Three small tubes per side, plus push-pull output EL84. One tube was for the phono pre-amp, one for the tone circuit, and the last was the AF amp / phase inverter.

I chopped out the phono and tone tubes, but found the plate voltage on the AF / PI tube started to creep up on me (265 volts or so, when it should have been 215). I don't know if it was enough to stress it, but I preferred to try to keep things close to spec. I put a dummy load (big resistor) where the missing tubes used to be, and everything seemed to fall back in line. I won't swear this is a good approach, but the amp seems to suffer no ill consequences.

I guess my point is that if you choose to eliminate tubes while you're cleaning out spaghetti, you might alter the operating voltages coming off the power supply.

[Eli Duttman]

I don't think I have any use for the phono stage in this amp, but the tone controls are important.

Your wish is my command. I've attached a Max Robinson design. 2 'X7s per channel and a DC heater supply are a bit much for tone controls. However, the cathode follower triodes can be replaced by DC coupled ZVN0545A MOSFET source followers, as described in MOSFET Follies. Be certain to use Sovtek the 12AX7LPS (a true 7025 equivalent, with a hum bucking heater) in the tone control "holes", so AC heating is quiet.

Make the signal processing circuitry totally defeatable. It should be in circuit only when absolutely necessary. Also, when in use, the tone control circuitry should insert between the source selector switch and the volume control circuitry.

[gofar99]

Greetings, That is a most interesting tone control circuit. Just out of curiosity, was it done in a PSpice variant or by trial and error. Just at a first glance it seems to have a number of intertwined feedback loops rather than the more common one or two local ones. I also wonder if the gain is sensitive to output loading. A neat design in any case.

Good listening
gofar99