70v Tap on Tube PA amp as a Recording Output?

[tablebeast]

I convert a lot of PA amps for guitar use and I was wondering about an alternate use for the the 70V output tap that seems to be present on most of these PAs. Can I use this tap for a line level recording output? I imagine I would have to pad it down, but is this possible? Specifically I am working with this Bogen amp right now that has a completely separate 70V secondary and I was wondering if I could use this as a balanced output for recording purposes? If so what do I need to do to knock it down to line level? Here is a link to the schematic http://richardstevens.home.mindspring.com/images/bogen_CHB20A.jpg

 

[JohnRoberts]

Not a big deal but from the schematic the 70V tap is not floating or balanced. It looks like you can't lift the ground either, since the amp is taking feedback from that tap.

70V will be more than you need, it would be easier to pull from a low impedance speaker tap. It looks like the low impedance stuff could be floated if you want.

JR

 

[tablebeast]

Oh yeah, I will be removing the feedback from the 70V tap as part of my mod. If I do that can I set it up as a balanced output or is the lack of a center tap preventing that? I've done plenty of the Garnet-style speaker-level padded to line level outputs using usually the 16 ohm tap. I was really just wondering if the 70v was somehow better for this purpose as I never use it for anything on these PA conversions. Seeing it listed as a 500 ohm output on one amp made me think it might be better suited for this use. Seeing the separate secondary on this amp made me think I could get a balanced output (if I removed the feedback).

 

[JohnRoberts]

It doesn't need to be center tapped to be balanced, if floating, and the following termination is balanced, signal should swing symmetrically about ground.

You will want to pad it resistively to get level down. You may need to look at power dissipation in those pad resistors, if you keep values low for low source impedance.

JR

 

[tablebeast]

It doesn't need to be center tapped to be balanced, if floating, and the following termination is balanced, signal should swing symmetrically about ground.

Cool, that's what I thought. So, without the feedback or ground attached it is floating.

You will want to pad it resistively to get level down. You may need to look at power dissipation in those pad resistors, if you keep values low for low source impedance.

So, I guess I should experiment with values. Any ideas of where I should start?

 

[tablebeast]

I found this simple circuit used to pad a 70v output down for a VU meter. Is this a good place to start for a recording output?

 

[JohnRoberts]

Yup.. but you might want lower source impedance.

If you scale down the resistor values you may need to use higher wattage resistors. E^2/R or (70x70)/R will give you typical dissipation in Watts.

JR

 

[tablebeast]

you might want lower source impedance.

According to the same page I got that pic from at 10 Watts the 70v on my amp is about 500 ohms, so should I just change the 1k to a 470ohm? 2 Watts should be plenty for all three resistors, right?

Also, I haven't used separate output taps simultaneously before. Do these taps (the 4-8-16 ohm speaker output tap and the floating 70V) interact with each other? If so, how? I was planning on having a 10ohm, 25Watt power resistor switched in across the 8ohm tap when no speaker is plugged in so that it could also be used without a speaker. How would this affect my 70V output?

 

[JohnRoberts]

[quote author="tablebeast"]

you might want lower source impedance.

According to the same page I got that pic from at 10 Watts the 70v on my amp is about 500 ohms, so should I just change the 1k to a 470ohm? 2 Watts should be plenty for all three resistors, right?

Also, I haven't used separate output taps simultaneously before. Do these taps (the 4-8-16 ohm speaker output tap and the floating 70V) interact with each other? If so, how? I was planning on having a 10ohm, 25Watt power resistor switched in across the 8ohm tap when no speaker is plugged in so that it could also be used without a speaker. How would this affect my 70V output?[/quote]



for that pad the load on the amp or power is all three resistors in series combined,, so (70x70)/(3.9k+3.9k+1k), So 70v loaded by 9k is around a 1/2W total. If this is too complicated just use the original values, a 1k source impedance won't kill you unless you drive longer runs.

------

The load on all taps combine so you can use as few or as many as you want as long as the total load doesn't exceed what the amp is capable of. just like hanging multiple speakers off a normal amp.

JR

 

[tablebeast]

OK, I'm starting to get it now.
Let me more fully explain what I am building before I go any further. I have these two rack-mount Bogen amps that are similar to the CHB-20A. The difference being that the model I have, MU-10 uses the same tubes (6EU7 and 2x6GW8) and has the same output transformer, but has a tube rectifier (6V4), is cathode biased and has a Microphone input transformer. Judging by the model number (MU-10) it is probably conservatively rated at 10 Watts, but I figure on 13 watts once I get the new design in there.

Using this amp as a platform I am keeping all the tubes the same and building a 'Marshall 18 Watt' style amp. I found a project that had already done a lot of what I wanted to do here: http://richardstevens.home.mindspring.com/guitar/index.html

So I want to use this design as a starting point but modify it for my purposes. My changes are:

1. XLR input (Using included Bogen Microphone input transformer. 125ohm/250ohm input impedance switch)
2. XLR balanced output. (Using floating 70v tap padded down)
3. 2x Hi-Z inputs, 1/4" on front panel. (switched jacks to allow Mic, Single, Dual, and Mixed inputs).
4. Modified Marshall 18 Watt circuit. (MODS: phase inverter 'starvation' control, tremolo replaced with parallel gain circuit, 'Vajra' variable SE mode, and a 10 ohm/25 Watt Power resistor across the 8ohm tap for speakerless operation.)
5. 1/4" unbalanced speaker-level, attenuated-line output.
6. 1/4" dual mono speaker-level, attenuated headphone output. (Line and headphone fed from the same pot).
7. Grounded IEC socket.

 

[tablebeast]

There is another one of these MU-10s on ebay right now for a decent price. It is not the rack mount type like I have, but instead the tabletop version. Same exact guts though. If anyone wants to get this one and make a similar device to what I am cooking up I say welcome aboard. The Mic input transformer it needs is a standard type and should be fairly easy to find a compatible one. I've had Newcomb, RCA, Webster, and Bogen all compatible with this 9-pin octal layout. I'll be documenting my efforts on an upcoming blog once I get it all sorted out.
http://cgi.ebay.com/BOGEN-MU-10-10-watt-tube-amp-guitar-harp-MULLARD-VALVO_W0QQitemZ120284923134