Preamp Valves

[VikingBlues]

I was reading an article about amps in general. A particular part in the section on Preamp valves / tubes caught my attention.

It mentioned the commonly used 12AX7 (ECC83). It referred to this as a "high gain" design. They mentioned earlier Fender amps being fitted with a lower gain 12AY7. My interest was further aroused when it mentions SRV favouring the 5751 (another low gain substitute).

It sounded to me a great idea to replace "sizzly compressed distortion" (their words) of hotter preamp valves for what was described as "more punch and clarity" to be experienced with low gain valves.

I've certainly always been impressed by the clarity of SRVs sound even when there appears to be a high level of overdrive. Is that clarity in part due to these valves?

The Fender VibroChamp XD has a 12AX7 so it is making me wonder .....

Anyone done a valve change with this sort of resulting tone change in mind or just generally had experience of low gain preamp valves?

[vancouverois]

I am looking for tubes for my old amp, I found this equivalence on the internet.

ECC83 = 12AX7 = Gain Factor of 100
ECC82 = 12AU7 = Gain Factor of 19
ECC81 = 12AT7 = Gain Factor of 60

The best is to ask a professional about tube swapping, I've read on a professional amp repair page that testing a tube is a tremendous ordeal to do it the right way under real conditions.
The text isn't in English otherwise I would have put a link.
Not sure about SRV using a particular type of tube.
Unfortunately many internet gossips/trends made the tube screamer quite famous by using SRV name.
Some "gurus" make money from these trends

[VikingBlues]

Here's a link which gives gain ratings od the various types of tubes / valves:
http://thetubestore.com/gainfactor.html

I notice the 5751 (that the Magazine article mentioned in regard to SRV) is not as extreme a change as the others.

The link also, quite rightly, points out your amp may not have a tone that you like with a different tube type. Having said that a tube isn't the most expensive bit of kit in the world amd it's supposed to be easy enough to swap back (is it?).

On the plus side I believe the XD is a Cathode bias amplifier and shouldn't require any adjustments and will work with a wide range of tube plate currents, as the circuit is "self adjusting" and is considered plug'n play when it comes to replacing the tubes.
http://thetubestore.com/fender-vibro-champ-xd.html

The SRV connection with 5751s seems a widely accepted connection on the internet
for example - http://tubedepot.com/5751.html
I hadn't heard about it before. Hopefully the article, which was in the UK Guitarist Magazine, has been written with knowledge of actual facts. Who knows!
BUT ... I've never opened up an amp before, so I might just end up chickening out anyway .... I do like the sound a lot as it is already. Well ... I do like the clean voicungs a lot .. not that keen on the overdriven amp voicings (Luckily my particular model of the amp has a cleaner than normal set of Blackface settings - I did a side by side test with a new XD in a local shop).

[ratfinkdan]

VB, i'm far from a tube guru but have changed tubes in a few of my amps, changing preamp tubes dosen't require any adjustments or re biasing and they are easy to change and rechange if you don't like the results. You just need to be carefull to pull the tube straight out, might take a little gentle wiggling side to side to get it to start out and when you re place them again, carefully, don't force it into the socket also use a towel or latex gloves or something , you don't want to touch the tube with your bare fingers cause the oil from your skin can cause the tube to burn out pre maturely. Power amp tubes may need to be re biased depending on the amp, if your replacing the power amp tubes with a tube with the same rating your usally pretty safe with out re biasing. Fender rates their tubes ( GT tubes) and there is a ratiing number on the tube so again if you replace the tube with one that is in the same rating your usually good. I know different tubes do make some difference in the tone of the amp although sometimes better sometimes not, but again its pretty easy to swap them out, hope this helps a little bit. Dan

[HalfBlindLefty]

Mhh, see the first few Superchamp posts I did.
I tried a Mullard ECC83 and right now have a JJ 12AX7 in the Champ.
Cybie has a mix of a Mullard ECC83 and a J&J 12ax7

[Kalle]

Hi Guys
If you want to know more about Valve Guitar Amp design check this http://www.valvewizard.co.uk/.
About the Fender VibroChamp XD, SuperChamp , they has not Valves in the Pre-Amp section.
The Pre-Amp section has Operational Amplifier IC´s
(check this schematics: http://support.fender.com/schematics/gu ... ematic.pdf
http://support.fender.com/schematics/gu ... ematic.pdf)
The 12AX7 ( dual triode) valve is used as driver and/or Phase Inverter the Power Amp sections.
I.e. all the distorsion sounds is made in the "signal processing IC" section.
Changing the 12AX7 valve to a 5751(or 12AT7 etc) will probably not change anything as the gain in the power amp is defined by a feedback loop.
/Kalle

[VikingBlues]

Thanks for pointing this out Kalle. As it happens I've not done anything about the valves on my VibroChamp and am not really wondering about doing that anymore.

I'll be honest and say the schematics are gibberish for me! I'll just take the message from the words you've written!

[tytlblues]

This is the only thing I have done to my Deluxe Reverb, I can say I do like the subtle difference that I hear. The amp used to have a certain harshness at times, and I find a it a bit more mellow now.....also could be that I don't play it that much and the speaker is just finally getting broken in ?



Pull out the V1 normal channel preamp tube for more preamp gain in vibrato channel

We’ll start with saying that this is a must have mod. It is so easy to enable and disable that it can hardly be called a mod. If you are like most players and only use the Vibrato channel (reverb, tremolo, the brigth cap and the extra gain stage), you should pull out the V1 tube. This is the preamp tube for the normal channel which you are not using. Vice versa; If you’re using the Normal channel, you can pull out the V2 tube. All AB763-similar circuits (Deluxe Reverb, Super Reverb, Pro Reverb, Twin Reverb, Vibroverb, Vibrolux) are designed so that signal is leaking between the two channels. The amp will play louder at the same volume knob setting when pulling the V1/V2 tube that you’re not using. The stronger signal will push the second gain stage (V4 tube) harder and give you increased sustain, compression and harmomics. This mod does not change the amp’s clean headroom.

This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps.

Replace the 12AT7 PI tube with a 12AX7 or 12AU7 – Less clean headroom.

Very practical mod at practice and low volume environments. This mod reduces the amp’s clean headroom and you’ll achieve sweet spot at a lower volume. You’ll notice that the amp gets looser and with less attack. Tips: If you pulled the V1 12AX7 tube you may use it as V6 phase inverter.

12AX7 as phase inverer tube will give the most effect out of this mod. 12AU7 will be in between 12AT7 and 12AU7.