TAD7025WA's vs. Genelex T12AX7:

[ivan H]

You are absolutly right about the supporting circuitry, I did not mention it because I felt in a guitar amp preamp stage it was not as big of a deal, as maybe to power stage.

Yes, you are right. Tubes are very forgiving, so will happily go on working under less than ideal circumstances, especially small signal (pre amp) types. Even power tubes, though to a lesser degree will handle less than ideal operating conditions. Cheers

 

[Inspector #20]

Maybe try a 5751 in position 1. A 5751 is essentially a ruggedize 12AX7 (developed for avionics applications) that has an amplification factor of 70, as apposed to a 12AX7"s amplification factor of 100. So it has 70% the gain of a 12AX7. A NOS one would be your best bet, they are quite available. Hope this helps. Cheers

I have the Tube Amp Doctor 7025WA's in stock and thay have a gain rating of '3' vs. the T12AX7's rating of '10.'

I think he might mean the updated ones that were just released at the last NAMM show (2018 update).

Marshall DSL40CR 40-watt 1x12" Tube Combo

Oh! I didn't know that!

 

[Inspector #20]

I've used NOS 5751's (RCA's) to tame V1 in a couple of amps that were a bit too hot in the past. A very good way to go.

Thanks for the tip. I will scour my boxes!

 

[Inspector #20]

I have several sets of Marshall branded tubes...all JJ's I think???

 

[syscokid]

Maybe try a 5751 in position 1. A 5751 is essentially a ruggedize 12AX7 (developed for avionics applications) that has an amplification factor of 70, as apposed to a 12AX7"s amplification factor of 100. So it has 70% the gain of a 12AX7. A NOS one would be your best bet, they are quite available. Hope this helps. Cheers

Yes yes yes! I third or fourth this advise.

For a $50 NOS tube, this is a good deal. 5 years ago - these tubes were selling for $25 - $30. Supply is getting very limited.
NOS JAN GE 5751 (1980s) SPRING SALE 2 for $84! | KCA NOS Tubes & Amplifier Repair

A modern 5751 will be around $20 to $25. IMO, stay away from JJ preamps tubes in a high gain Marshall circuit. And if the seller gives you the option of testing for the lowest noise or microphonics, for a few extra bucks, I would go for it. That first tube in the chain needs to be as quiet as possible.

 

[67plexi]

I have had bad luck with the RCA black plate 5751 going micro phonic after 20-200 hours, good luck with the JAN 12AY7

 

[RVA]

You can get a JJ5751 for $13 and a Sovtek for $23. Here is what an Amazon reviewer said about the Sovtek in a DSL40

I use 5751 preamp tubes in almost all of my amps typically in the V1 or V2 slot. On my Marshall DSL 40 this tube occupies V2. Combined with a JJ ECC823 in V1, the DSL 40 is now a completely useable amp with both channels, rather than the Red channel being way over the top gain.

 

[RVA]

Yes yes yes! I third or fourth this advise.

For a $50 NOS tube, this is a good deal. 5 years ago - these tubes were selling for $25 - $30. Supply is getting very limited.
NOS JAN GE 5751 (1980s) SPRING SALE 2 for $84! | KCA NOS Tubes & Amplifier Repair

A modern 5751 will be around $20 to $25. IMO, stay away from JJ preamps tubes in a high gain Marshall circuit. And if the seller gives you the option of testing for the lowest noise or microphonics, for a few extra bucks, I would go for it. That first tube in the chain needs to be as quiet as possible.

I like EH preamp tubes in my Marshall JVM

 

[Inspector #20]

You can get a JJ5751 for $13 and a Sovtek for $23. Here is what an Amazon reviewer said about the Sovtek in a DSL40

I use 5751 preamp tubes in almost all of my amps typically in the V1 or V2 slot. On my Marshall DSL 40 this tube occupies V2. Combined with a JJ ECC823 in V1, the DSL 40 is now a completely useable amp with both channels, rather than the Red channel being way over the top gain.

Wow, RVA! That's great!

 

[Inspector #20]

I have a ECC83-MG, a bunch of EC833 and some 7025-WA's here, so I will have to go shopping!

 

[Inspector #20]

Found this:

 

[Inspector #20]

5751 looks like the ticket!

 

[Inspector #20]

Look at the ECC81 gain factor of 60....are all these tubes compatible in a mix and match fashion???

 

[Chubbles]

You won't damage anything by changing tubes...so long as you perform the swap correctly.

And yes, they do "color" the amount of gain and to be to some degree, the tone as well.

12AX7 Comparison of Current Made Tubes | Amplified Parts

I've used that Amplified Parts chart as a preamp tube guide for years.

 

[ChasFred]

Amazon has a good selection of Russian made 5751's. I was just there looking.

 

[ChasFred]

I've used that Amplified Parts chart as a preamp tube guide for years.

I live about 4 miles away from Amplified Parts. No store to shop just will call.

 

[Inspector #20]

I live about 4 miles away from Amplified Parts. No store to shop just will call.

I buy all mine from there...I am going to trim this gain a bit.

Either a 5751 or a ECC81

 

[Chubbles]

Amplified Parts come to my Philly guitar shows and leave catalogs, but they don't sell anything.

(Sorry about the double pic above, I can't fix it)

 

[syscokid]

The parameters of a 5751 are very similar to a 12AX7 (aka ECC83, 7025, ECC803, CV4004), except for the gain factor. 12AT7's and 12AY7's are supposedly interchangeable with a circuit designed for a 12AX7. But I know you like to run your amp hard, and since I lack the proper knowledge to say for sure, I would be cautious about using them.

If it were me... I would start with one 5751 in V1. Then move it over to V2. Maybe one in V1 and V2. But, I bet a 5751 in V1 is all you might need...

 

[syscokid]

I like EH preamp tubes in my Marshall JVM

I like EH tubes, too. For a modern day tube manufacturer/supplier, they have a real good track record of being reliable and low noise.