Varimu related question

[chrissugar]

I heed the help/experience of the tube/varimu gurus related to the input and output transformers for this circuit.
What ratio, and what current (the output transformer) should be rated for optimal operation and low distortion (1%) at +16dBm out.
If you know an existing product that fits the conditions it would be helpful.

http://img.photobucket.com/albums/v292/chrissugar/VARIMU/varimu-x.jpg

Thank you in advance.

chrissugar

 

[gyraf]

If you know an existing product that fits the conditions it would be helpful.

An EAR660? :razz:

 

[NewYorkDave]

I'd like to have a look, but Photobucket is blocked here at my job.

 

[chrissugar]

[quote author="gyraf"]An EAR660? :razz:[/quote]

Errrr... Funny :green: Ok, another answer please. :grin:

chrissugar

 

[analag]

Strange configuration, for two reasons IMHO (1) it's better to have four triodes sourcing current through two and (2) cascode operation means higher output impedance, effectively creating the need for a higher stepdown ratio, thus less gain.

analag

 

[rafafredd]

COME ON...

where´s the sidechain???

 

[PRR]

> What ratio, and what current (the output transformer) should be

Pretty picture.

What does it do?

If you've done your homework, you know the gain-cell's output impedance and current for any gain condition. At "maximum" gain reduction (whatever you choose that to be), current will be minimum, and very low. It will be hard to get a high-power output, at the moment when you need big output.

The cascode makes the usual triode-trouble a non-issue.

With those 33K resistors, output current will be quite small. Without doing your math, my guess is that you can NOT get deep reduction AND high output. You either need to compromise, beef-up, or add a post-amp.

> thus less gain.

Gain is cheap.

Power is expensive.

Power becomes a real problem when you expect 100mW-500mW while the gain-cell is working at less than 0.3mA per side. Low current, you need high voltage and thus very high impedance to get decent power. If deep GR takes the gain cell toward 0.3mA per side, yet you can load it in 100K, then you have 33V 0.3mA peak or 5mW +7dBm maximum output.

Limiter gain-cell design starts with the output loading.

Six bottles.... I have to wonder if a sextet of '39/44 supercontrol tubes wouldn't work as well. In a mini-7 base they call it 6BA6.

 

[chrissugar]

PRR, thank you very much for your thoughts.
From what I read the designer went this route to exclude any capacitor from the signal pass. Here are the specifications from the manufacturers site:
SPECIFICATIONS
Input and output impedance 600 ohm nominal, balanced and floating.
Gain (max) 20dB
Input and output normal operating level +4dBm
Maximum output level (1% distortion) +16dBm linear
Attack time 0.2 to 0.8ms variable
Release time 0.3 to 5s variable
Release time (classical music setting) up to 25s for sustained high levels.
Compression ration 1:1 to 10:1 adjustable

This is the product:

chrissugar

 

[analag]

Seems my post has been edited. Very peculiar indeed.

analag

 

[gyraf]

Rowan, could you decribe what?

 

[analag]

Jakob , I posted Chris's schemo and I don't recall making any changes to anything, but change it did.

analag

 

[Guest]

[quote author="chrissugar"]PRR, thank you very much for your thoughts.
From what I read the designer went this route to exclude any capacitor from the signal pass. Here are the specifications from the manufacturers site:
[/quote]
There are loads of these on Dave Gilmours houseboat/studio
These have been further mod'd
We?ve done some modifications to his 660s. These input attenuators - we put these multi-turn pots on and did a power supply mod as well.

Check out the interview -
http://www.tapeop.com/magazine/bonuspdfs/philtaylor.pdf

 

[gyraf]

Hmm.. wierd.. :?