Ebay electronic parts seller gets mad because I ask questions.

[gary350]

There is a seller on ebay that buys old amps then strips them for parts. He sells the parts on ebay. Some of his ads are wrong, miss leading and are short on information. He strips good working Tube Amplifiers and sells the parts.

The guy has a transformer for sale. The subject line says, Vintage Power Transformer 6L6 5U4 436V. The ad says, The transformer is stamped Part #15123 306003. 120V input. Windings are 426-0-426VAC, 5.4VAC, 6.75VAC, 6.5VAC. Very clean with no rust or corrosion. This large transformer used a 5U4 rectifier and powered several 12AX7's and 2 6L6's's.

I did the math 426 x 1.414 = 602 VDC. All of my tube books say 600 volts is max voltage. Typical B+ voltage is 250 to 450 VDC for a 6L6GC tube..

I told the seller I did a web search for that part number but nothing comes up. Are you sure you have your numbers correct 426 VAC would be 602 VDC. How many watts is the transformer?

His reply was. What do you need it for?

I told him what I needed it for and he said, Mine your own business!

Seller is, vintage_amp_parts

 

[ericgibbs]

hi,
Look at this pdf for the 5U4G rectifier, 450Vrms max in , with a cap filter , +470Vdc out.

 

[alec_t]

I did a web search for that part number but nothing comes up

If this is a vintage part it pre-dates the Internet; so it's quite likely a web search won't find it.

426 x 1.414 = 602 VDC. All of my tube books say 600 volts is max voltage

When you allow for the voltage drop across the (selenium / tube??) rectifier the DC output could be considerably less than 600V.

 

[gary350]

hi,
Look at this pdf for the 5U4G rectifier, 450Vrms max in , with a cap filter , +470Vdc out.

I see what your saying the 5U4 is rated 450 VDC at 225 ma. the filter cap has to be over rated you don't want to push it to the limit until it goes up in smoke. But that does not mean the power transformer is rated 450 VDC at 225 ma. I have worked on many circuits where the power transformers was lower voltage and a lower current than the rectifier tube.

 

[ericgibbs]

From the d/s its says for 450Vrms input you would expect ~+470V dc output from the 5U4G. So when its used in regular valve amp it would have additional smoothing and voltage division for the different amplifier sections.

So basically its a transformer thats designed for a valve amp.

How do you plan to use it.??

 

[gary350]

From the d/s its says for 450Vrms input you would expect ~+470V dc output from the 5U4G. So when its used in regular valve amp it would have additional smoothing and voltage division for the different amplifier sections.

So basically its a transformer thats designed for a valve amp.

How do you plan to use it.??

I need a power supply for a Dynaco 70 amp I am going to build from scratch. I have several circuit drawings for 6L6GC tube push pull Tube amps that are almost identical to the Dynaco 70. The 6L6 is a tetrode the EL70 is a pentode. Pentode is better less hum, less noice, less static, much better for audio. It would be easy to put the EL70 is a 6L6 circuit or a 6L6 in an EL70 circuit only difference is 1 extra wire grid #3. My Dynaco 70 circuit drawing is not clear but I think I can figure it out by compairing it to 6L6GC circuits then taking voltage readings after the amp if finished and compairing voltage and amp readings to the Tube Manual Data. I can do the math to make it right. There are 3 versions of the Dynaco 70 that I know about I am not real sure what the differences are. I know the early version ran a lower B+ so the amp was not 35 watts per channel. The newer version one has 2 preamp tubes the other version has 3 preamp tubes. The Dynaco 70 circuit drawing that I have shows only 2 preamp tubes.

 

[ericgibbs]

hi gary,

A 6L6 is a tetrode, not a triode.

Are you going to use a dual cathode valve rectifier or solid state rectifiers.?

If the latter, the +HT smoothed will much higher than a valve rectifier.

 

[Jon Wilder]

Me personally, I would not use a valve rectifier on that hot of a B+. Sure 602V is the unloaded B+ but it's only gonna drop to about 550-560V loaded, which IMHO is still way too hot.

Moreover, the screens need to be considered as well since 6L6 screens aren't rated to handle above 450V as it is.

I would go for a transformer with a B+ winding of around 350-0-350VAC with the necessary 5VAC and 6.3VAC windings.

 

[gary350]

hi gary,

A 6L6 is a tetrode, not a triode.

Are you going to use a dual cathode valve rectifier or solid state rectifiers.?

If the latter, the +HT smoothed will much higher than a valve rectifier.

That is a mistake anyway I am not sure what I was thinking when I typed that triode it is a pentode too. The 6L6GC is a pentode too both tubes have 3 screens. The only difference is 6L6 has the screen connected internally to the cathode. EL34 the #3 screen is connected to pin #1.

 

[Jon Wilder]

That is a mistake anyway I am not sure what I was thinking when I typed that triode it is a pentode too. The 6L6GC is a pentode too both tubes have 3 screens. The only difference is 6L6 has the screen connected internally to the cathode. EL34 the #3 screen is connected to pin #1.

This is absolutely incorrect. The 6L6GC is a beam tetrode. It does not have a suppressor grid. It has beam forming plates between the screen grid and the plate. These beam forming plates are internally tied to the cathode.

 

[Electroenthusiast]

.... miss leading ....

That ebay seller even Amuses me!

 

[gary350]

This is absolutely incorrect. The 6L6GC is a beam tetrode. It does not have a suppressor grid. It has beam forming plates between the screen grid and the plate. These beam forming plates are internally tied to the cathode.

Page 344 of the RCA Receiving Tube Manual show it has 3 grids. The book does not say pentode it only shows the pic with 3 grids. It says that it can be used as a triode amp. It is a class A ampplifier.

Page 114 in the GE book it is called a Beam Power Amplifier. Again it has a pic with 3 grids but the book does not say pentode.

Page V21 in Amateur Radio Handbook Class A Beam Power Amp. It says nothing about pentode here eigher. Pic shows 3 grids.

Interesting. I always assumed it was a pentode because it has 3 grids.

This is new to me, tell me how it works?

The Dynaco 70 uses EL34 tubes grid 3 is connected to pin 1 the amp wiring has pin 1 connected to the cathode just like 6L6 is shown in the tube drawing. I have read the 6L6GC and the EL34 are interchangeable.

 

[mvs sarma]

perhaps ,a seller need not be technical and expert to answer our queries.

 

[mvs sarma]

Page 344 of the RCA Receiving Tube Manual show it has 3 grids. The book does not say pentode it only shows the pic with 3 grids. It says that it can be used as a triode amp. It is a class A ampplifier.

Page 114 in the GE book it is called a Beam Power Amplifier. Again it has a pic with 3 grids but the book does not say pentode.

Page V21 in Amateur Radio Handbook Class A Beam Power Amp. It says nothing about pentode here eigher. Pic shows 3 grids.

Interesting. I always assumed it was a pentode because it has 3 grids.

This is new to me, tell me how it works?

The Dynaco 70 uses EL34 tubes grid 3 is connected to pin 1 the amp wiring has pin 1 connected to the cathode just like 6L6 is shown in the tube drawing. I have read the 6L6GC and the EL34 are interchangeable.

the wiki reads and I quote "Power output pentodes, also referred to as "Beam Pentodes",[3] include the Sylvania (and possibly GE) 6CA7 version of EL34, 6V6GT and the EL84/6BQ5. Sometimes the word "pentode" was avoided, in names such as "beam power amplifier" or "beam power tube", at a time when the pentode patent was an important issue."