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Help me 'start up' my new project; 7868 SE stereo amp

M

mp53

Active Member
Hello, So I want to build a single ended amp modeled after D.C. Gillespie's Modified Magnavox 8604-00 and Wharfcreek's thread on his 7591 SE project.( Help me 'dial in' my latest creation; 7591 SE stereo amp) What I have in my collection is a pair of SE output transformers that came out of a console using 7868's, I've calculated them to be about 2800 ohm primary impedance at 8 ohms ,although I'm going to recheck that, fairly heavy weight to them, so I think I'm good there. Also, 5 pairs of 7868 tubes. A power transformer out of another console that used EL84's single ended, puts out about 325 volts static , no load. A couple of chokes, smaller one and a bigger one that came out of various consoles, Various 12ax7's. 7868 tube sockets. I'll be driving a pair of Jensen designed ultraflex cabinets with Dayton ps 220-8 drivers, just don't have a preamp of any kind . I think I have enough for a good starting point. My first question is can I use the 6ca4 power tube or do I need a bigger rectifier tube ? I'd like to use the smaller one because I will have to buy a 5 volt heater transformer. Thanks for any ones help. Regards, MP
 
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datasheet says plate and screen at 300v wants a 3k trafo so you're well within the neighborhood there. The bigger question is if the power transformer can deliver the current. A pair of single end EL84 needs about 100ma total for stereo, looks like a 7868 stereo will idle about 140 per pair and at full output you're right near 180, plus whatever the driver pulls. With a 12ax7, call it maybe 2ma.

Idle current will be just within the limits of a 6CA4, full power is beyond it. I'd also suspect the transformer itself just won't be up to the task. Manufacturers are generally not known for grossly over-building them.
 
I see, I do have two other power transformers , one out of an organ using a mono amp with 6878 PP and a Fisher 400 stereo using also 7868 PP. Both would be running at over 400 volts B+. That would seem like a lot to tame down to make work.
 
if you could find something out of a push-pull stereo EL84 amp that ought to get it done. Right voltage and enough current.
 
mp53 said:
Hello, So I want to build a single ended amp . . . using 7868's . . . just don't have a preamp of any kind. I think I have enough for a good starting point. My first question is can I use the 6ca4 power tube or do I need a bigger rectifier tube ? I'd like to use the smaller one because I will have to buy a 5 volt heater transformer.
Click to expand...
If you're concerned about the current capability of the 6CA4 you could always use a pair of them, assuming you have enough heater current.

Another alternative, if you don't have a 5v winding, is to use a "cockeyed bridge" with a 6.3v damper diode. They provide a slow startup and they're cheap and plentiful. That's what I'm using for the direct coupled amp I've been breadboarding. I'm using a 6AX4GTB but there are others that can also be used.
Cockeyed Bridge with 6AX4GTB.jpg

One interesting thing I've noticed about the 7868 / 7591 in SE mode is that, while the power output is rated at 11w, the distortion at that level is listed as 13%. Most tubes list their maximum output at 10% and occasionally lower. This is the only tube I've seen which lists power output at over 10%. I've always wondered how much power it makes compared to other tubes at comparable distortion levels.

But from a practical standpoint, it seems to me that the ability to drive an amp to full power is not that important in most cases.

It takes 2x the power (a 100% increase) to create a 3db increase in sound. That's the equivalent of moving 39" closer to your speakers. So, if your amp is producing only 50% of full power, a 3db increase is possible if you go from 50% to 100%. But that's only if you listen to the amp at full volume.

If your amp is only being driven to 75% of full power, the difference between that and full power is a 33% increase. So that will result in a whopping 1db of additional volume but, again, that's only if you listen to the amp at full volume. At normal listening levels (typically no more than 1w) you'd only get a tiny fraction of a db more. Probably the equivalent of tilting your head forward while listening at your normal level.

One cool thing about the 7868 is that, like a 6BQ5, it's very easy to drive. I modded a little SE 6BQ5 amp a while back that doesn't have an input tube, just the outputs. I normally use it with a variety of preamps but I was surprised to find that it will actually work quite well with only a line level input.
 
well, I may have that tube in my collection. Otherwise your saying ,this present power transformer may be sufficient?
 
@FlaCharlie
What is the advantage, if any, of the cockeyed bridge vs having a regular FW CT power supply with the damper in series with the B+? Am curious.
 
Slow startup. Can also ground the center tap, flip the diodes around and land them all at the plate of the damper, comes out the same.

Either way if the transformer doesn't have the capacity it's not going to matter how stout the rectifier is.
 
Thanks. Just wondered. I understand the use of a damper for slow start. Was curious why the reversed layout of components. I have plans to use a damper for slow start and a little voltage drop in the B+ of some mono power amps. I have used dampers in pairs as replacements for traditional dual diode rectifiers in DIY project. Using a pair of 12CT7's in an amp project at the moment. I think more robust than a 5AR4 but a bit more complication with two tubes.
 
I just copied the configuration that Eli Duttman (who used to post a lot on AA and DIY) suggested. I think his initial suggestion didn't use a tube there but a high voltage Cree Schottky diode.

He configured it like that because, at the time, the Cree diodes had recently been introduced and were quite expensive. So this configuration saved money by using only one of them. He later posted a version with the tube. I like the idea of having a slow startup and I'm pretty much a copy and paste builder so . . .
 
mp53 said:
well, I may have that tube in my collection. Otherwise your saying ,this present power transformer may be sufficient?
Click to expand...
Just saying that if you choose your operating points based on what the PT can supply, the compromise, in terms of power output, might not even be audibly noticeable. Especially with the 7868 since the distortion is higher than a typical tube at full power output. So you may not even want to squeeze that much out of it if the distortion is that high.

The damper diodes generally have very low voltage drop too, but then so does the 6CA4. Though the cockeyed bridge may also give you the best of both worlds - SS rectification with a true slow start.
 
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