500ohm OT tap on RCA PA head

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lassoharp

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I have an old RCA 2X 6V6 15W PA head with a 500ohm tap on the OT that I have a couple of questions about.

First - Does anyone know the original intended use of this tap?  It's common either as a 500 or 250ohm tap in nearly every RCA PA schematic I've seen.  I've taken some rough measurements and I'm getting ~ +8 to +10dbm output into a 500ohm load with average input drive voltage.  Perhaps they used it to feed a bigger dedicated distribution amp ?  The particular one I have (no available model#) also has a dedicated 124ohm/70.7V distribution tap(assumedly providing full power output at that impedance)  .

Second - My bigger concern was whether it was intended that the 500ohm tap be used as a line level feed in conjunction with standard speaker connection.  In other words, is there risk in using just the 500ohm tap without having a speaker load simultaneously connected using one of the low taps?

The online catalog sheets don't mention any specific use of this tap.

This is not the exact amp I have, but is similar enough for a reference

http://www.theused.com/manuals/rca/rca_mi-12287_12295.pdf
 
For large systems with disparate speakers, they ran 500/250 and used appropriate step down transformers at the speakers.  Predates the modern 70 volt system standard.    Load is a load. 

If it were a broadcast series amp, you might see it used as high level program distribution (NBC did this in NYC, rather than using Prog amps) or for disc cutting at 500 or 250 ohms.  My Gates monitor amps have similar taps, and work fine as preamps if I want to go there.  Likewise some of the RCA Prog amps have speaker taps for on the spot monitoring or emergency usage.  Most broadcast consoles gave provision to use the Mon amp as an emergency Prog amp at the throw of a switch; another reason for lots of taps.    Collins Radio didn't even bother with speaker output Z's on their Mon amps, just gave 600/150 and assumed use of step downs or high-Z speakers. 
 
Thanks for the good info.  I wasn't aware of the older system. This one must be intermediate as it has both the 500r and 124r/70v taps.
 
> with average input drive voltage.

What is "average"??

> I'm getting ~ +8 to +10dbm output into a 500ohm load

Two-6V6 is 10 Watts clipping any day. 10 Watts is +40dBm.

Before the VU meter (note the older 0.006 watt reference is still cited), 500 ohms was "the" universal impedance. If you didn't have a right impedance, you could always cobble something with a XX:500 transformer in the middle.

You may put 250-1K ohm 20W load on the 500 ohm tap and run it into clipping.

BTW: 10 Watts in 500 ohms works out to just 70.7V. So you are set to drive any supermarket sound wired for 70V and 10W or less.

There are no "Line Inputs" which should really be fed +40dBm. Not to mention that most "600" boxes today are really >10K. And will smoke at 70V 0.14A input. And when smoked, the 6V6 OT will be unloaded and possibly arc-over. Any real engineer knows you put a several-DB pad between. A 3dB pad will still put +37dBm into a load, and will keep the amp loaded within 2:1 even if someone trips-out the patch.

> 124r/70v taps

70V on 124 ohms is 40 Watts, which aint no two-6V6 chore.

124 is 35V, apparently half of the 70VCT winding.

> If it were a broadcast series amp

Yup. But the referenced MI-12287 amp doesn't even have a "flat" setting. (And the 500 winding is end-grounded, whereas we often want our long-lines balanced.)

Streamlined case ought to go real fast.
 
PRR said:
> with average input drive voltage.

What is "average"??

> I'm getting ~ +8 to +10dbm output into a 500ohm load

Two-6V6 is 10 Watts clipping any day. 10 Watts is +40dBm.

Before the VU meter (note the older 0.006 watt reference is still cited), 500 ohms was "the" universal impedance. If you didn't have a right impedance, you could always cobble something with a XX:500 transformer in the middle.

You may put 250-1K ohm 20W load on the 500 ohm tap and run it into clipping.

BTW: 10 Watts in 500 ohms works out to just 70.7V. So you are set to drive any supermarket sound wired for 70V and 10W or less.

There are no "Line Inputs" which should really be fed +40dBm. Not to mention that most "600" boxes today are really >10K. And will smoke at 70V 0.14A input. And when smoked, the 6V6 OT will be unloaded and possibly arc-over. Any real engineer knows you put a several-DB pad between. A 3dB pad will still put +37dBm into a load, and will keep the amp loaded within 2:1 even if someone trips-out the patch.

> 124r/70v taps

70V on 124 ohms is 40 Watts, which aint no two-6V6 chore.

124 is 35V, apparently half of the 70VCT winding.

> If it were a broadcast series amp

Yup. But the referenced MI-12287 amp doesn't even have a "flat" setting. (And the 500 winding is end-grounded, whereas we often want our long-lines balanced.)

Streamlined case ought to go real fast.


Sorry for not stating the situation more clearly -

"Average" was +4dbm from a CD player > 40db pad > Hi-Z input with gain control set just below onset of noticeable distortion - I was approximating the signal level the circuit would see under normal conditions for the intended use - as a line utility amp.  The padding was taken care of on the front end with the assumption that it will always be used.

> Two-6V6 is 10 Watts clipping any day. 10 Watts is +40dBm.

Yes, I expected that

You may put 250-1K ohm 20W load on the 500 ohm tap and run it into clipping

This is something I'm still uncertain of - whether extra padding is still needed on the output at that(20W) wattage for overdrive situations given the heavy padding already on the front.  ?

70V on 124 ohms is 40 Watts, which aint no two-6V6 chore.


No, it ain't.  My omission - the amp originally used 6L6s - I stuck 2 6V6's in there. 10W is enough overkill, 20+ would've been worse.  This amp is a project for a friend who doesn't have any dedicated studio tube gear and wants to put this head to use as a utility amp.  It's somewhat collectible so rebuilding something with a more sensible output may not be a good option. I was trying to come up with something relatively hackless.

With the 40db pad in front I was getting ~1.8 - 2.0VAC across a 500r terminating resistor feeding a Hi-Z line input. This seemed a reasonable level and the signal was fairly clean.

I'd asked about the 500r distribution set up because 40 W into 500r would be 141V and I didn't know if that tap was intended to provide that sort of output level.


 
lassoharp said:
You may put 250-1K ohm 20W load on the 500 ohm tap and run it into clipping

This is something I'm still uncertain of - whether extra padding is still needed on the output at that(20W) wattage for overdrive situations given the heavy padding already on the front.  ?

It's really a matter of noise and user self control.  If you think he's prone to smoke the following device, then pad the output accordingly.  If you find 'normal' operating levels seem to have a noise advantage with one pad versus the other, pick the better.  All the Gates stuff had a 20 dB output H pad that kicked in when you switched to emergency program operation.  They are always burnt up from being under-spec for  wattage.  Me, I might try a 10 dB output pad for a bit of isolation and see how it then feels.  Then he can only put 1W into the next piece. 

lassoharp said:
I'd asked about the 500r distribution set up because 40 W into 500r would be 141V and I didn't know if that tap was intended to provide that sort of output level.

Sure it is.  The output trans on a BA-6A does 10W at 500/600 ohms.  Same for the Gates SA-39, the Stalevel, the Collins 26U, etc. 

You can sometimes get a sonic upgrade using something like a UTC LS-33 500:speaker step-down on a piece that has 500 output taps, but is of lesser sonic merit.  The onboard does less stepping and damages the output less, the higher quality follower does the rest.  More iron sound, but sometimes still less damage. 
 
If you think he's prone to smoke the following device, then pad the output accordingly

Very likely - heavy overdrive tastes.  I may go with the arrangement that's on my 39B - 2 6db output pads with the 1K control pot coupled with the front end pads.  That might give a better range for driving the front end with some extra safety for the next stage input.


The onboard does less stepping and damages the output less,

Always a concern.
 
6V6 in a circuit scaled for 6L6 will not reduce power.... just burn-up 6V6es and maybe the OT also.

> heavy overdrive tastes

For a speaker? Or for a "color box" at line-level?

Then hang a BIG 10 ohm resistor on the 8 ohm tap, and give him the 4 ohm tap via a 560 ohm resistor.

The 10 ohm keeps a load on things. This should be rated twice the nominal sine-RMS power, because "heavy overdrive" is square waves and at essentially the same peak voltage that is twice the power of a sine wave test-tone.

The 4 ohm tap at say 30 Watts (typical for older 6L6 amps) is 11V RMS 15V peak. These are at the upper end of what we expect modern gear to put out, and therefore should not make much smoke. The 560 gives something less than 30mA max, not far above the 10mA that nearly any chip pin can take. (2K would be safer, but there just might be some true-600 stuff around.) And a short on "his" connector won't trouble the amp in the least.

> given the heavy padding already on the front.  ?

No matter how much padding you put on an input, some fool will find a bigger source to reach overload.

If you actually use more padding than any source can overcome, the tube amp will run so far below max power that it won't strain nor add color.

Go into the pot-input phono jack. Since it is a pot no pad is needed (you may need to change to an audio-taper pot to control huge inputs). Let him hammer the 25W output to "47 Watts" of gross goodness. Then pad the OUTput until smoke stops coming out of the next box in the chain.
 
You want a Herzog. The "American Woman" tone. Randy Bachman was running guitar to Champ-amp, no speaker, then into another amp. But the Champ died a lot. Gar Gillies knew that Champs don't die that often, and discovered that Bachman was running it no-load. Gar knew how to get that sound without that destruction. He not only added the permanent load, he INcreased the gain at the front of the Champ (you want overload? here's OVERload!!). Then added an output pot because the screamin' Champ's 4 ohm out can hammer a 12AX7 grid into blocking (and melt the newer solid-state inputs).

"the famous American Woman tone - it's sort of a one trick pony in this department, but a great trick no doubt with four distingishable dynamic range zones at the touch"
http://www.lynx.bc.ca/~jc/garnetAmps.html

The push-pull amp won't be the same sound but has its own overdrive charms.
 
For a speaker? Or for a "color box" at line-level?

Both, but from listening tests it's maybe an average guitar rig but it does that James Brown-Live 60's-hairy vocal-fuzzy PA sound like there's no tomorrow. I think he wants to get a little of that at line level.

Thanks for the 8-loaded 4 ohm out arrangement suggestion - I haven't tried that one yet.



PRR said:
You want a Herzog. The "American Woman" tone. Randy Bachman was running guitar to Champ-amp, no speaker, then into another amp. But the Champ died a lot. Gar Gillies knew that Champs don't die that often, and discovered that Bachman was running it no-load. Gar knew how to get that sound without that destruction. He not only added the permanent load, he INcreased the gain at the front of the Champ (you want overload? here's OVERload!!). Then added an output pot because the screamin' Champ's 4 ohm out can hammer a 12AX7 grid into blocking (and melt the newer solid-state inputs).

"the famous American Woman tone - it's sort of a one trick pony in this department, but a great trick no doubt with four distingishable dynamic range zones at the touch"
http://www.lynx.bc.ca/~jc/garnetAmps.html

The push-pull amp won't be the same sound but has its own overdrive charms.


Ah yes.  Closest I ever got to that sound was using an old Ibanez rackmount delay unit that had a 'pull-mic' feature on the input gain knob - drove the amp input mad and gave an almost e-bow like quality.  Other way was using the Ritchie Blackmore trick of running into the Hi-Z mic amp of a tape recorder. Probably similar circuits.



 
PRR said:
6V6 in a circuit scaled for 6L6 will not reduce power.... just burn-up 6V6es and maybe the OT also.

hang a BIG 10 ohm resistor on the 8 ohm tap, and give him the 4 ohm tap via a 560 ohm resistor.

The 10 ohm keeps a load on things. This should be rated twice the nominal sine-RMS power, because "heavy overdrive" is square waves and at essentially the same peak voltage that is twice the power of a sine wave test-tone.

Wise you are, sir. 
 

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