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 Post subject: Amplifier question
PostPosted: Apr Sat 20, 2019 6:50 pm 
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I have the following amp I use with my Hammarlund SP-600 JX-17 which is directly driven by the output transformer of the SP-600.

Attachment:
SP-600 booster amp.PNG
SP-600 booster amp.PNG [ 7.49 KiB | Viewed 572 times ]


My question is this.

Will the typical pro audio balanced signal be able to drive the amp to full output as is or would I need to add a triode stage before each 6V6.

Reason I ask is because if the pro audio output can drive the tubes directly that would make building a stereo amp much easier.

Not anything I'm looking to do right now, but I want to put this away for my future use and/or to help a forum member in the future who wants an easy to build amp.

The maximum output of pro audio gear is +24dBu which converted to voltage equals 12.28Vrms and 34.72Vpp.


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:18 pm 
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Well, we can start with the tube specs:
https://frank.pocnet.net/sheets/127/6/6V6.pdf

At Ep = 285 and I(0) of 70mA, the tube wants to see a peak grid to grid voltage of 38. Your 34.72 P-P gives almost twice that.

What is the tube voltage and current for yours?

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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:22 pm 
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Think the current and voltage is similar, but I don't remember as it's been a long time since I measured it.

Think this may have been covered before, but if so I've forgotten and am confused about what peak voltage grid to grid means.

Now that output voltage is the absolute maximum specified for normal pro audio levels with +4dBu being normal.


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:27 pm 
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What are all the references to "pro" supposed to mean ?

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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:29 pm 
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SP-600 is Super Pro 600, a good receiver.

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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:30 pm 
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That means professional audio such as bands, studios ETC...


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:32 pm 
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Tube Radio wrote:
That means professional audio such as bands, studios ETC...
What does that have to do with an SP-600 ? ? ?

It's a communication receiver, with audio bandwidth that's intentionally restricted.

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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:38 pm 
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Leigh wrote:
Tube Radio wrote:
That means professional audio such as bands, studios ETC...
What does that have to do with an SP-600 ? ? ?

It's a communication receiver, with audio bandwidth that's intentionally restricted.

- Leigh


I merely stated that's what the amp was built for so people would have some sort of idea why I built the amp originally.

The idea is to eventually build another amp or pair for stereo HI-FI use.


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:47 pm 
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TR,
A Tung-Sol data sheet gives 38 V peak grid to grid for max output with 285 V dc on the plate and screen. 38 V peak gives 26.8 RMS, which is over twice the 12.28 RMS +24 dBu signal. As pixellany indicates, the exact dc voltages in the amp would have to be known to make complete analysis. Some amplification will be needed between the line and the 6V6 grids. Voltage gain of 2.2 would be the minimum, four or five would be better so the equipment providing the line signal will not have to be operated at maximum gain.

The same transformer or circuit that provides some amplification of the line signal will need to also produce the 180 degrees out of phase drive for the 6V6 grids relative to ground as apparently your SP-600 output does.

WB5HDF


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 8:59 pm 
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So I would need a low gain dual triode such as the 6SN7 and would be driven by the balanced output of a pro audio device or other device with a balanced output.

Yes the SP-600 does provide the phase inversion in its output transformer as the transforemr has two 300 ohm windings with the center of both normally shorted together on the rear panel which is what I use as the ground connection for the amplifier, although the amp does work equally well without the ground connected given the SP-600 output transformer is not tied to the receiver's ground in any way.


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 9:17 pm 
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Tube Radio wrote:
So I would need a low gain dual triode such as the 6SN7 and would be driven by the balanced output of a pro audio device or other device with a balanced output.

Yes a low mu triode like that will do it.

WB5HDF


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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 9:50 pm 
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Tube Radio wrote:
I have the following amp I use with my Hammarlund SP-600 JX-17 which is directly driven by the output transformer of the SP-600.

Attachment:
SP-600 booster amp.PNG


My question is this.

Will the typical pro audio balanced signal be able to drive the amp to full output as is or would I need to add a triode stage before each 6V6.

Reason I ask is because if the pro audio output can drive the tubes directly that would make building a stereo amp much easier.

Not anything I'm looking to do right now, but I want to put this away for my future use and/or to help a forum member in the future who wants an easy to build amp.

The maximum output of pro audio gear is +24dBu which converted to voltage equals 12.28Vrms and 34.72Vpp.



Your 200 ohm cathode resistor should be 7 to 10 watts , not 3 watts .

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 Post subject: Re: Amplifier question
PostPosted: Apr Sat 20, 2019 10:22 pm 
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Why?

I measure 15.85 volts across the 200 ohm resistor which gives a current of 79.25mA and a wattage rating of 1.26 double that for safety margin is 2.5 watts. 3 watts is the closest higher value.

Now you said 7-10 watts.

So given a doubled power for safety

That would be 3.5-5 watts.

If I do 3.5 watts for the 200 ohm resistor I get 26.458 volts across the cathode bypass cap which is rated at 25 volts. I also get a current of 132.29mA being drawn and the power transformer is only rated at 70mA on its HV winding.

Now I do have two 10 watt 100 ohm resistors in series as that's what I had on hand when I built the amp.

B+ to the output transformer and screens is 295.5Vdc.

This does have one error in it. I built it with a Hammond transformer which has a 115 Vac primary and it is run on 120Vac so that causes the HV to be a little higher which increases B+ and that is why I needed the 560 ohm resistor.

I cannot remember if I played around with using the 5Vac winding in series with the primary to lower the voltage and didn't do it because I didn't have the right resistor to get the correct B+ or if I didn't think of that at the time.


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 Post subject: Re: Amplifier question
PostPosted: Apr Sun 21, 2019 2:14 am 
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infzqi wrote:
Tube Radio wrote:
So I would need a low gain dual triode such as the 6SN7 and would be driven by the balanced output of a pro audio device or other device with a balanced output.

Yes a low mu triode like that will do it.

WB5HDF
Do you have schematics to copy, for resistor and cap values? There are a few different layouts for drivers/phase inverters. The engineers had reasons for doing it different ways with various tubes.
The one called 'long-tailed pair' comes to mind for this circuit, not looking for a large increase in gain. That design basically splits the signal equally to either side of a dou-triode.

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 Post subject: Re: Amplifier question
PostPosted: Apr Sun 21, 2019 2:45 am 
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Tube Radio wrote:
Why?

I measure 15.85 volts across the 200 ohm resistor which gives a current of 79.25mA and a wattage rating of 1.26 double that for safety margin is 2.5 watts. 3 watts is the closest higher value.

Now you said 7-10 watts.

So given a doubled power for safety

That would be 3.5-5 watts.

If I do 3.5 watts for the 200 ohm resistor I get 26.458 volts across the cathode bypass cap which is rated at 25 volts. I also get a current of 132.29mA being drawn and the power transformer is only rated at 70mA on its HV winding.

Now I do have two 10 watt 100 ohm resistors in series as that's what I had on hand when I built the amp.

B+ to the output transformer and screens is 295.5Vdc.

This does have one error in it. I built it with a Hammond transformer which has a 115 Vac primary and it is run on 120Vac so that causes the HV to be a little higher which increases B+ and that is why I needed the 560 ohm resistor.

I cannot remember if I played around with using the 5Vac winding in series with the primary to lower the voltage and didn't do it because I didn't have the right resistor to get the correct B+ or if I didn't think of that at the time.


Every 6V6 PP amp i have ever seen has had a 10 watt cathode resistor . If you never drive the amp into clipping , have a tube red plate or become leaky 3 watts might work

http://www.bunkerofdoom.com/xfm/DYNACO_01/Dynaco_01.pdf

http://www.bunkerofdoom.com/xfm/ACROSOU ... d_1955.pdf

See page 9 on the acrosound 6V6PP schematic

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 Post subject: Re: Amplifier question
PostPosted: Apr Sun 21, 2019 1:39 pm 
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With the few tube amps I've seen I've never seen a 10 watt cathode resistor. Even in the push pull 6L6 20 watt output section of a Magnavox CR-198C console.

Westcoastjohn, should I need a phase inverter I have a schematic for one I've used a few times which works good. Got it from the schematic of a GEC 912 PLUS amp.

EDIT:

The amp was built into the case of an RCA Volt Ohmyst JR which I had bought with the intent of restoring it since I had one just like it in my teens, but the meter being unrepairable prevented me from doing that so it was sacrificed for the amp as it already had the necessary octal sockets.

A 3" fan on front mounted where the meter went turning slow being powered by a 1/2 wave rectifier serves to remove heat given how compact the amp is.

One idea I just had is if I ever build a pair for HI-FI is to perhaps use defunct service grade or below VTVM chassis to build the amps with a separate one for the 6SN7. The lower impedance of the 6SN7 driver stage would negate any issues connecting the driver to the amps with a few feet of shielded cable.

I could either do the phase inverter design for an unbalanced input or I could do a balanced design where the two triodes are wired the same to accept a balanced input.

Either way I will need two 6SN7 tubes and a B+ of around 250Vdc.

I do have a single Jensen 15" and three Jensen tweeters from an Emerson console.

If I can find two similar Jensen speakers and good tweeters perhaps I could make an all tube powered system provided around 10 watts is enough to make the Jensens go loud and sound good doing it.

The Jensens wouldn't necessarily need to reproduce the real low bass as a powered sub can easily handle that duty, but it would be nice to have just the Jensens.

With the amps each being in their own case I could set each one inside the rear of the open back speaker cabinet with the driver being set elsewheres.

I would of course only use a commonly available VTVM that is not good enough for modern use so as not to gut something that is still usable.

But it doesn't have to be a VTVM. Can be anything with a couple octal sockets and room for two transformers or I may even go with a tube rectifier such as a 5Y3 which would require three octal sockets, but would increase the heat inside the case.

One change I would make is this.

Given the Edcor output transformer has both 4 and 8 ohm taps I will most likely install two 1/4" jacks so that I can plug in either four or 8 ohm speakers without having to rewire anything. Or I may use one jack and a switch or perhaps do like Magnavox did on some of their tube amps that could run external speakers. I could always find four Jensen 15" speakers and some tweeters then use those on the 4 ohm tap of the transformers either making four speaker cabinets or incorporating them all into two cabinets.

Ideally I would want to find a second set of the Jensen speakers I already have without junking a console to get them. The only reason I got the ones I have is because someone on the forums here was junking a console, but forget exactly why and I asked if he was selling the speakers. Got them for a decent price. They currently reside in a cardboard box I used to test them with.


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 Post subject: Re: Amplifier question
PostPosted: Apr Mon 22, 2019 11:56 pm 
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If you're not dead set on using a pair of 6v6 tubes, you might look at something else, like the 6BQ5, that requires much less drive voltage and will probably be just about right without the extra triode gain stage.


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 Post subject: Re: Amplifier question
PostPosted: Apr Tue 23, 2019 12:15 am 
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I could try those.


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 Post subject: Re: Amplifier question
PostPosted: Apr Tue 23, 2019 8:36 pm 
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classicelectronicsguy wrote:
If you're not dead set on using a pair of 6v6 tubes, you might look at something else, like the 6BQ5, that requires much less drive voltage and will probably be just about right without the extra triode gain stage.


A related option worthy of experimentation would be the 6DZ7. It's essentially a pair of 6BQ5s shoved into a single bottle on an octal base. They typically sell for $8 to $12 on eBay when they come up, which is less than a pair of NOS 6BQ5s would run.


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 Post subject: Re: Amplifier question
PostPosted: Apr Tue 23, 2019 11:12 pm 
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So basically it ties the two screen grids together and also the two cathodes together, right?

That would be a nice choice.

Imagine a single tube 10 watt push pull amplifier driven by a balanced source.


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