C22 Warning - tube preamps

[dewickt]

Here is a factory warning about tube preamps, this can include other brands like Marantz 7C and Citation.

 

[nj pheonix]

Thanks for the heads up Terry.
I never would've given it a second thought.

 

[62caddy]

Terry...does that apply to others like the C11, 20 and MX110?

Thanks.

 

[Pio1980]

I'd assume it applies to any class A preamp that doesn't have power linked delayed reset crowbar automute at the output end. Certainly most vintage tube preamps don't.
This should be common knowlege but isn't, something I'd thought about as I added a crowbar power interruption delayed reset automute to a mod'ed PAS.

 

[sKiZo]

power linked delayed reset crowbar automute.

I'll bet you've been waiting to say THAT for a while ... <G>

 

[Pio1980]

I'll bet you've been waiting to say THAT for a while ... <G>

Oh, I've probably already mentioned it somewhere sometime. Easy to do with a '555 timer chip and dpdt mini-relay.

 

[62caddy]

Makes sense but why would Mc single out the C22...

 

[damacman]

This is valuable information.

 

[Pio1980]

Did a websearch for simple add-on automute modules expecting multiple responses and came up empty.
Simply stated, mine is powered by the AC heater bus thru it's own small ps with light filtering for a fast response weak reservoir capacity. A 555 timer powers a small dpdt relay to open crowbar muting contacts across preamp output terminals after ca 30 seconds, a diode aids fast discharge of the time constant cap when power drops below a given level to reset the timer when the relay is unpowered and clamps the preamp output to ground.

 

[twiiii]

Crown had a device schematic for turn on delay and protection of their amps published in their DC series owners manuals. I used to build them to protect speakers in Discos when the mixers would emit a loud thump when turned off using MC 2300's. In other discos where there were many amps and tons of processors. mixers, wireless mic receivers, etc. and tape decks. Sources were on one breaker, Mixers and processors on another, and amps in pairs on other circuit breakers. Then they were color coded and were turned on in sequence and off in reverse. Eventually manufactures designed sequencing power control devices. That we used for big systems. I don't remember who did it first. But I know Atlas, Soundilier and Midland Atlantic made sequencing power controllers. There was another company and thats all they did was make sequencers, power control modules ,Noise filters and Power back-up for emergency AC power loss.

 

[nj pheonix]

I'm a little at a loss. I understand what i'm not supposed to do. I'm a little in the dark why. What condition happens from the C22 to the DC coupled amp that causes possible speaker damage?

 

[sKiZo]

Tubes4hifi (Latino amps) has a time delay kit that would probably do the job ... 8A rating, so it's pretty industrial, and it should work on any tube gear.

http://tubes4hifi.com/bob.htm#TDR

PS ... I get a hefty thump out of my speakers if I forget and turn the SS pre on after the amps are up and running. Solution there? Don't do that ... <G>

 

[Pio1980]

Tubes4hifi (Latino amps) has a time delay kit that would probably do the job ... 8A rating, so it's pretty industrial, and it should work on any tube gear.

http://tubes4hifi.com/bob.htm#TDR

PS ... I get a hefty thump out of my speakers if I forget and turn the SS pre on after the amps are up and running. Solution there? Don't do that ... <G>

Similar approach to mine but different application, it delays application of B+ in a tube amp until cathodes are hot.

The thump is somewhat the warning is about, a tube preamp can swing the output of a DC coupled solid state amp hard lock to lock during power up/power down. So can some solid state preamps. Then the order of turnon/turnoff matters but all bets are off during a power interruption or brownout.

 

[nj pheonix]

So if i understand this right. If the preamp is on and powered up before amp is turned on and if amp is turned off before the preamp is turned off, there is no problem. Is that right?
So the protection circuit, I'm missing the operation too. When does it come in to protect the amp. It delays the turn on of the amp (after the preamp is powered)?
Why would this be different than a SS preamp?

 

[Pio1980]

So if i understand this right. If the preamp is on and powered up before amp is turned on and if amp is turned off before the preamp is turned off, there is no problem. Is that right?
So the protection circuit, I'm missing the operation too. When does it come in to protect the amp. It delays the turn on of the amp (after the preamp is powered)?
Why would this be different than a SS preamp?

You have the order correct.
My automute works at the output lines of the preamp, not at the power supply. It protects against improper power switching, brownouts, and power interruptions. It's been a while since I made it, I think it's an original concept I made up after looking at some 555 chip timer circuits and not copied from someone else.

 

[sKiZo]

... the order of turnon/turnoff matters but all bets are off during a power interruption or brownout.

Got that covered ... I have a non-latching adapter on my tube stuff that required a manual reset after a power blip.

TRC 90033 Shockshield White Portable GFCI Plug with Surge Protection

That gives the amps time to discharge properly and not take a hit. The SS stuff will cycle fast, but that's not really an issue, and we're past the "thump stage" if I wait a bit before resetting the adapter to pass power.

I still say this is one of the best things $20 investments you can make for your system, and peace of mind.

* Doesn't protect against brownouts, just those quick blip blip hits that can overload tube equipment that hasn't had time to discharge properly.

 

[c_dk]

Makes sense but why would Mc single out the C22...

The C22 was initially re released as original faithful to the original collectors unit.

Right into the modern audiophile creed of having amp bandwidth of DC to light.

Old designs had bandwidth limitations, the MC2105 has a switch to take the network out of the circuit.

 

[dcgillespie]

I don't know about the re-release of the C22, but the original C11/C22 does not have a turn on thump, nor a response to DC, as its main output is capacitively coupled from a cathode follower output stage configuration. Therefore, even with a SS power supply, the turn on charge of the output coupling cap only ramps up at the rate that the final 12AX7 begins to conduct. Quick power burps are a different matter of course, where sKiZo's answer is perfect.

I believe that the re-issue C22 uses an extra tube over the configuration of the original designs, and it may be that the main output is now obtained from an anode follower configuration. If that is the case, then that configuration, combined with a SS power supply, and no delayed output muting, would definitely spike the output of a DC coupled power amp to power rail output levels at a cold turn on.

Dave

 

[Pio1980]

All versions of my tube preamp had cathode follower output, even so I designed and added the automute when I went from a Stereo 70 to a solid state amp with it as insurance for the speakers.
If I dive back into it, I may just reestablish the Vorhis Last PAS line stage if I can find the skiz for it online.

 

[62caddy]

To be clear - is the phenomenon being discussed here is unique to tube preamplifiers only? (Assuming those without delay circuitry and such).

What about the turn-on THUMP from SS preamplifiers? That's different?