Tube Load Impedance Questions

[silverfox]

Happy Saturday Forum Members,

I've got an OT that was used in a 7868 push pull amp. I want to use it with "2" 6L6GC tubes.

Looking at the tube data sheets the 7868 looks for 5500 ohms plate resistance for a AB push-pull pair. 6L6GC wants 6600. At 40 watts output will this be an acceptable match.

I also do not know the "AB" type, (1 or 2) for this power amp. The plate voltage for the output is reduced to about 430 VDC

Data Sheets attached for your viewing pleasure:

A763 power amp design schematic, (only 2 tubes will be used):



Alms to the Gurus'

Thanks Silverfox.

[pdf64]

Unless you already know the primary and secondary impedances your transformer was designed for, I think that it would be best to check the turns ratio.
Feed a small VAC into the primary and measure what the transformer changes it into on the secondary.
Pete

[Chuck H]

What Pete said.

You made it a point to stipulate "2" 6L6's would be used. Was the original 7868 amp a four tube amp? There are ways to make many transformers work with many different tube types. And, WRT guitar amps, the load only needs to be within safe operating parameters. We're not shooting for a certain wattage with a certain frequency response at a certain distortion figure. Ergo, less critical parameter. In fact you may even find that something other than ideal is best.

FWIW 5500 is fine for a pair of 6L6's. Or 6V6's or EL34's or 6550's...etc. Fender ran 6L6's at various voltages from 4k to 6.6k throughout the models and years. Also FWIW the primary load can be manipulated with the secondary load too. If, for example, you wanted to use your 5.5k OT for a quad of 6V6 tubes, you could do that. 2750 might be a better load for a quad of 6V6's so you could just half your secondary load to reflect that lower figure on the primary. Why four 6V6's instead of four 6L6's? Because it's probably a 50W OT. An OT's fidelity is idealized for it's intended use, but that all goes out the window with guitar amps for the reason I stated above. So it's more a game of rating and ratio.

[potatofarmer]

Is there any way you could be persuaded to try 7868s or 7591s? They really are a fantastic tube.

[silverfox]

Is there any way you could be persuaded to try 7868s or 7591s? They really are a fantastic tube.

I still have the original tubes tucked safely away. Due to the fact they run about $40-$70 or more each and may not have come back into production yet, I wouldn't want to use them.

By the way, several times this week end I tried getting back to respond or especially offer thanks for all assistance. consider it done should I not respond timely. So several times this week-end I tried responding but kept having a sort of white out experience. Thought I was having a flash back or something...

Okay, the history of this amp is: The Iron came out of a McGohan 35 watt tube based PA amp. I tried wiring it up for guitar use and while it sounded okay I don't think I had it properly matched with the effects unit so at times and settings it sounded thin. After that and disappointments with amp purchase upon tone questing journeys I decided to build my own tube amp. That was a little over two years ago.

Since that time I have used that Iron in a couple of short lived builds and currently may swapp it an X-chassis. Just don't know how well matched.

The output transformer in question supported "2" 7868 tubes in push pull at around 400 volts as I recall. Have the sch But! Before I wire it into my next build I wanted to get some other opinions on compatibility. It sounds like the consensus is: It should work okay for experimental purposes. It has worked with EL34's in the past.

Reason for Questions: As a novice to tube amp construction and design I wanted some opinions before wiring it into an X-chassis design. (experimenters chassis). I've got the chassis started and will build my next design on it. I got the idea from another forum and the ones they build are very versitile with screw terminal for mounting parts. Prototype building for tube amps...

Alms to the Gurus',
Silverfox.

[Chuck H]

I'm not much for building in module form because it doesn't usually allow enough consideration for layout. Idealizing layout and lead dress is something that should be built into a design. Guitar amps are very high gain devices. Even the ones that don't sport cascade preamps. Unless you have experience with layout and lead dress you risk instability. And if you DO come up with a working circuit you then need to halfway disassemble it to rebuild it properly. Screw terminals won't do in the long run.

The original PA amp probably wasn't designed to amplify a 200mV signal or expected to be clipped. You could use those transformers to build a known design. If done right there is a much lower risk of complications due to layout and lead dress.

What was wrong with the EL34 amp that you decided to build a whole different amp from scratch? What "versatility" is offered by the amps being built on this other forum? As far as I know an amp needs to sound good to you, be loud enough, have the EQ features you need, overdrive gracefully and be reliable. If your wanting more exotic features from a build like reverb, channel switching and effects loops I would definitely go with a known circuit and layout to avoid noise and stability problems that a less than perfect layout, lead dress and ground scheme can cause.

[silverfox]

X-chassis and amp labbuilddesign questions answered

IScrew terminals won't do in the long run; You could use those transformers to build a known design; What was wrong with the EL34 amp that you decided to build a whole different amp from scratch? What "versatility" is offered by the amps being built on this other forum; I would definitely go with a known circuit and layout to avoid noise and stability problems that a less than perfect layout, lead dress and ground scheme can cause.

Thanks Chuck.
My reasons for building the amp became twofold, that is: Want the ultimate tone and retaking up my original electronic interests. These Forums have become my University in the field of: "Tube Amplifier Design and Construction" I look only for the brightest and best instructors. (you all should feel honored. I hope you guys get my dry sense of humor. That's all it is at times).

The first design of the EL34 amp was a power amp only. It worked but there was some distortion arising from the PI most likely. The power amp was a known design. The other Forum is comprised of what I've gathered, a few very knowledgeable Gurus that I don't have a good personality type fit with. And some are outright arrogant. I learned a great deal from that Forum but due to a lack of interest in the posts I made I couldn't get a good response and in some cases I would tear apart a working amp to get design experience and when I ran into a problem there would be no follow up information leaving me hanging. That's why I went searching for new sources.

On that site: EL34world.com there are a huge number of builds with pictures and schematics from which I drew my learning experience. Also the site operator offers parts for sale if you want to buy them and has good customer service.

I'm not plugging the site but want to make sure when I complain about it that a sufficient portion of the story is told.

The X-chassis. The experimenters chassis I'm putting together is just a Peavey Road Master chassis. Lots of room and I'm leaving the power and output components in place as well as the layout. I will be using the power amp design, (hopefully with the Forums assitance) and hacking a pre amp from that. The screw terminals are used to quickly change component values without gooping up parts. The layout, due to the use of a chassis will be consistent with at tag board style so upon completion it should only need transfer to a perment media.

Questions?

Silverfox.

[Chuck H]

That seem fine. As long as the lead dress and ground scheme can stay in place once you've developed a "working" model. So... Why the change to 6L6's from EL34's? You did mention resurrecting the tinkering bug so I'll guess that's part of it. But I assume you already have EL34's to work with. I don't know that you have the 6L6's. It's really the pot calling the kettle black since I do build stuff and tear it back down now and then myself. And, of course, I have a box of tubes just waiting to be used.

[kg]

I wouldn't sweat the impedance. I use secondary taps as tone controls. This ain't no hifi!

[silverfox]

Tubes and More

That seem fine. As long as the lead dress and ground scheme can stay in place once you've developed a "working" model. So... Why the change to 6L6's from EL34's? You did mention resurrecting the tinkering bug so I'll guess that's part of it. But I assume you already have EL34's to work with. I don't know that you have the 6L6's. It's really the pot calling the kettle black since I do build stuff and tear it back down now and then myself. And, of course, I have a box of tubes just waiting to be used.

I too have a box of tubes and parts just waiting to be used.

6L6, EL34, 6550, these are the general battle ground. I typically build dual adjustable bias with a range that supports any of the above. I should be able to get an image up by the end of today.

Yes, ground schemes and wiring layout are important maxims to follow. I'm sure I'll be posting requests for debugging assitance at some point.

Out for the day,
Silverfox.