Another 5AR4 went on one of the VRD's...

[NOSValves]

Seems you and Craig are in agreement. Uh, has that ever happened before?

Hey everyone -- bookmark this thread!!

Maybe Parts Express got a bad batch of them. I had a situation similar to Mikes and went through four of them over two weeks time. I went to Rubys and haven't had a problem since.

The moon and sun are aligned in perfect order today. This will not happen for another 131.5 years.

[Erik Mandaville]

"Seems you and Craig are in agreement. Uh, has that ever happened before?"

Actually, yes. I remember once when crossover building was compared to the construction of an entire amplifier, and we both agreed that the two were very, very different in terms of complexity, time involvement, significantly greater number of connections for amps, etc.

Maybe you're right about a batch of Sovteks being bad. I used them in the Horus rendition I made, and had no problems at all. Speaking of Horus, I've GOT to get another pair of grid chokes for the modified Moondogs. I miss those amps sometimes, and the MQ grid chokes were an important part of their unique sound.

When I get done with the PP 6L6 amp I'm hoping to make some headway on this Thanksgiving, I want to do a parafeed 300B that will incorporate some of the original aspects of that design, and some of my own changes. 300Bs on both Klipschorns and Heresies were really something.

Did I read correctly in another thread you're selling your Peach? What are you going to use for a preamp?

Erik

[Erik Mandaville]

Thebes:

Those Bogens look great, by the way. Hexfreds should be great. I'm contemplating SS rectifiers in this latest project I'm working on, too, but haven't decided yet.

Erik

[thebes]

Hey Mike, been busy and frustrated with cars troubles (the work van,

not the new car). I'd like to get down your way some day.

Eric, thanks for the kind words, actually they look much nicer having

been painted and sitting on some nice cheery wood my brother put

together for me. They sound surprsingly detailed with a nice

soundstage. Nothing like some good iron, russkie power caps that

Craig likes so much, some orange drops etc. etc. This is

all thanks to MikeBSE of Jubilee fame who literally spent hour after

hour helping me as I tried repeatedly to blow and burn myself myself up.

Only quibble is they do have some SS steel to them, lacking a tube

rectifier. The hexfreds helped but we were unsuccessful in

getting some beefy Hammond chokes to play nice with them. If you

haven't done a diode rectified amp before (and I'm sure you probably

have) I think you'll ifnd that to be the case too. The freddies

made a hard to describe change over regular diodes, a little less grit,

a smidge more bass and a bit more authority is about the best I can do.

Bang for the buck I am very, very pleased and use them a lot. Who knew some old PA amps could sound so good.

Craig, of course, secretly thinks I am some jumped up tube

parveneau. I suspect however, he is working feverishly in his lab

with a mad gleam in his eyes, an insane cackle issuing forth as part of

his dementia as he plots and schemes to produce his own 100 watt

monster.

[Mike Lindsey]

Thebes, just let me know... there's always a room for you buddy!

BTW, got the 5AR4's today and am back up and running. I think my problem was caused by the amp switch box that Dr Who made for me. I didn't realize the tube amps needed a load on them when I switched back to them. I am pretty certain that is what caused the rectifier to blow.

As an added bonus, my Denon 2900 also came in the mail today. Still in the process of setting it up, but I think I'm gonna like this unit a lot. Got this one off of Agon for $350 shipped, and it looks brand new! Said it was manufactured in March of 03... who knows how long it will last, but at that price if I can get 4 or 5 yrs from it I'll be satisfied.

Mike

[Erik Mandaville]

"This will not happen for another 131.5 years."

I agree.

[DrWho]

Hey Mike, let's confirm that the amp selector thing didn't get ripped apart on the inside during shipping...did you ever get around to check on the "rolling"?

There are a pair of dummy load resistors in the selector that provide a speaker-like load to the amplifier not being played through the speakers. When you switch amps, the speakers and the dummy load resistors both get switched at the same time. There is no overlap in the relay and there is at most a 20ms delay during the transition:
http://www.alliedelec.com/Images/Products/Datasheets/BM/OMRON_ELECTRONICS_LLC/Omron-Electronics-Llc_Industrial-Control_8214042.pdf

All that to say, you should be able to switch between amps without needing to turn the tube amp on and off. Unless of course there is something wrong with "no load" for at most 20ms...I'm kinda hoping Craig will comment, but everything I've read says it's ok.

[Mike Lindsey]

Hey Who,

Got your email and will open up the box tomorrow. Hopefully, Craig will add his thoughts if he sees this thread.

Mike

[Erik Mandaville]

On this switchbox, does the amp that's not in use also have a signal being fed to it, or is the signal also switched from one amp to the other?

Erik

[tillmbil]

Wow, sorry to hear about the VRD troubles. I have been busy so I have not fired up mine for awhile. Just turned them on and they sound great as usual. Mullards AR4's, Tele fronts and GEC KT88's. This set up has been in them since my purchase over two years ago. Still cranking, and amazing all the listeners.

[DrWho]

On this switchbox, does the amp that's

not in use also have a signal being fed to it, or is the signal also

switched from one amp to the other?

Erik

Just looking at one channel (since both channels are the same) - there is amp A, amp B, the speaker, and a dummy load resistor.

When you select amp A, the 4PDT relay routes amp A to the speaker

and amp B to the resistor. When you switch to amp B, the relay routes

amp B to the speaker and amp A to the resistor. Both amps share the

same resistor and speaker, but they never power the same load at the

same time. In fact, there is at most a 20ms delay where no amp is

connected to anything during the transition.

[Gilbert]

Holy roasting rectifiers batman!... Does this mean I'll have to be hold my transformer [:$] while trying to listen to my new VRD's? STOP THE PRESSES, CANCEL MY VRD ORDER, I'm gong for the big sound, going for the BOSE.

[Erik Mandaville]

"When you select amp A, the 4PDT relay routes amp A to the speaker and amp B to the resistor. When you switch to amp B, the relay routes amp B to the speaker and amp A to the resistor. Both amps share the same resistor and speaker, but they never power the same load at the same time. In fact, there is at most a 20ms delay where no amp is connected to anything during the transition."

Right. I understand this aspect of the switchbox; however, what I'm talking about is the first stage, not the output stage. IOWs, there is amp A and amp B, and the box is designed to switch between the outputs of one or the other to either a speaker or dummy-load resistor. However, they both require an input, which is the aspect I was talking about. By your description, it seems that both amp A and amp B have a single input in common, which puts both amps in parallel with one another. So, if amp A has an input impedance of 100K, and amp B also has an input impedance of 100K, the resulting overall input impedance of the box with both amps connected would be 50K ohms. That is not a huge difference, but depending on the output impedance of the signal source connected to it, may possibly result in a less than ideal load.

What I was wondering is if there is also a switch on the INPUT end of the switchbox that allows for the signal to be routed to one amp or the other, where the input impedance of each amp remains independent of the other.

Erik

[Erik Mandaville]

"Does this mean I'll have to be hold my transformer "

Nah, just snug the screws down a bit with a nutdriver and screwdriver if they loosen over time. A simple mechanical thing that has to do with the physical construction of the type of transformer used, not the amp circuit. VRDs or Moondogs or Baldwin organ amp, transformer machine screws sometimes need a little attention.

Erik

[markgod]

With many people extolling the virtues of the Mullard 5ar4 . Why not try the groove tube 5ar4 , that Brent Jesse proclaims "It has the identical quality as the 1960's mullard" at a current production price. People have said that Brent Jesse is a reliable source for NOS tubes. The groove tube can be found for $21. Check out Brent Jesse web site for the above quote. www.audiotubes.com

[DrWho]

By your description, it seems that both amp A and amp B have a single input in common, which puts both amps in parallel with one another.

Wow. Amp A and amp B are seperate "inputs" and never connect to each other - what you propose would be an absurd and even impossible design.

EITHER [A is connected to the speaker and B is connected to the resistor] OR [A is connected to the resistor and B is connected to the speaker].

Attached is a picture of just one side of operation (you need this x 2 for stereo):

The purple, red, and blue jacks are the terminals on the relay. When amp A is selected, the Purple terminals are routed to the red terminals. When amp B is selected, the purple goes to blue. I assume anyone reading this understands how a 4PDT relay works.

(I have a feeling I'm going to have to explain that the same colored terminals are not connected to each other. The top purple terminal will only connect to the top red OR the top blue depending on the switch. Likewise, second purple to second red OR second blue and so forth....)

[fini]

I believe Erik is talking about what's fed INTO the amps (preamp/source). That isn't in your diagram.

[DrWho]

Well in that case, the output from the amps gets plugged into this device. It's a speaker level switcher, not a line-level one.

[Marvel]

Mike,

I think that what Eric is asking about is whether or not the amp with no load, still has a preamp signal feeding it. If there is no input, the 20ms won't be as critical, because the amp would pretty much be at idle and therefore no output. Do you understand? There owuld be no voltage swing across the output tranny.

Bruce

[DrWho]

There is never an amp with no load (except for the 20ms transition stage).

Is it possible that the tube amp has a signal driving its input during the 20ms transition? Yes. However, everything I've read indicates that this is a non-issue. In fact, there are even times during normal loudspeaker operation that the motors will exhibit "no loads" for short periods of time.

Btw, I hate using the term "no load" because technically the amplifier is seeing a 60,000 ohm impedance (or whatever the impedance of air is). "No Load' in my engineering classes means there is a short circuit (0 ohm impedance).

The box was originally intended to be used for those that have completely seperate HT and 2-channel signal paths. So in that case, both amps would be getting fed a completely different signal.

But if both amps were being fed the same input signal (say for comparing two amps against each other), then it still shouldn't matter. Even overdriving the preamp isn't going to cause either of the connected amps to exceed their output capabilities. Besides, you can usually connect at least 4 devices to a single line-level output if the gear is built within normal specs.

In other words, the box doesn't care what input the amplifiers see.