Saturday, January 1, 2011

Tube Showdown in VHT Special 6 Amp

Here are three videos that illustrate the sonic differences between three preamp tubes in the VHT Special 6 amp.

Harp players often change the preamp tubes in amps, almost reflexively... They want to quiet the amp down a bit to control feedback, and they want to warm up the tone slightly. All too often the harp player will rely on advice -- often bad advice -- gleaned from Internet harp forums or from other players. I wanted to do a test everybody could hear and make their own judgements.

I chose the modded VHT Special 6 amp because it is a Class A single-ended amp, which makes it a perfect test bed for a tube test. The circuit is simple yet sounds so sweet.

The tubes I chose were the stock 12AX7 that comes in the VHT amp, a NOS JAN Philips 5751, and a NOS GE 5965. The 5751 has about 70 percent of the gain of the 12AX7, while the 5965 (12AV7) has about 40 percent of the gain of a 12AX7. Both are thought to be warmer than the standard tube. The 5965 is quite similar to the 12AY7 tube many players use.

In these videos the audio is normalized to a common level, so you will not be able to hear the changes in volume from one tube to the other. I wanted too focus on tone rather than volume. NOTE: The back panel is off the amp in all three tests. Here are the amps setting for all three tubes. The pull boost was not on:

First, the stock 12AX7 tube: There is nothing wrong with the tone of this tube. I like its punchy, gutsy feel. You could leave this tube in the amp and love the tone. It is loud but just a touch bright without being overly harsh. Cuts through the mix like a hack saw.

As many of you know, the JAN Philips 5751 tube is my favorite preamp tube for harp, and I think it stands out here. It sounds fuller and warmer than the 12AX7, with a broader more complex tone. Breakup sounds more pleasing to me. It sounded as loud as the stock tube.

The 5965 tube had a rather large drop in volume, and the sound was kind of lifeless and un-dynamic. It was the darkest sounding of three tubes. In bigger, more powerful amps this may be okay, but in a 5- or 6-watt amp it just sacrifices too much punch and power.

Both the stock 12AX7 and the 5751 sound great in this amp. Either one is a good choice, but I give the nod to the 5751. What do you think?


Le lover bleu said...
This comment has been removed by the author.
Rick Davis said...

Yes, the differences are subtle, as you would expect. If you go to the Eminence speaker website, for example, you will find audio samples for their many different speakers. The differences can be very sublte indeed, but they are distinguishable across the Internet and worth noting nonetheless.

There are cerrtainly sonic differences between and among the three different preamp tubes I tried in VHT Special 6 amp. For some, the differences may be meaningless. For others, they are important.

Unknown said...

Hi Rick,
I have a VHT Special 6 amp with no mods. The stock 12AX7 tube sounds good in this amp. I have tried an Electro-Harmonix 12AY7 tube in it and it did not sound as good, less volume and not very musical. I also have tried an Groove Tubes Gold Series 5751 in this amp and it is the best sounding tube I have tried so far. It sounds full and punchy and musical. It also gives the amp the best vintage sound, very warm and full.
I am looking forward to hearing more about this amp. I am also wondering what is the best power tube for this amp?
Thanks, Robert

peter Moore said...

The first two sound the same to me.

The third one sounds lower than the other two.

That's what my ears hear.

Is this a stock amp or have you fiddled with the electronics?

Rick Davis said...

Hi Robert-

I think the original 6V6 power tube that comes in the amp sounds very good. Really, this is an exceptional amp that was very well thought out and tested.

In my VHT Special 6 I use a JJ/Tesla 6V6S power tube, which sounds slightly warmer and darker than the stock tube. Mike Fugazi, the harp player who first discovered the Special 6 amp and wrote about it, also likes the JJ 6V6 tube.

Search around this blog a bit in the recent posts and you will find lots of good info on this amp. I love it.

Rick Davis said...

Peter, the amp was modded for harp by Bruce Collins of Mission Amps in Denver.

Info is here.

Unknown said...

"The 5965 is quite similar to the 12AY7 tube many players use."
That's simply not correct, Rick. The 5965/12AV7 is quite a different tube to the 12AY7. Although the gain is similar there are other factors at work. It has a very low plate resistance compared to the 12AY7 and the higher current draw will alter (i.e. lower) preamp voltage and change the bias of the stage. A 12AX7 has a plate resistance of around 60-80K. the 5751 has a plate resistance of around 58K, the 12AY7 around 25K and the 12AV7 4.8K- lower even than the 12AU7 (6.2K to 7.7K).
That explains why the tone is much darker, and the headroom is much increased.
There's more to any preamp tube swap than just the gain factor. It's relatively easy to alter the gain and bias of preamp stage by swapping out 1 or 2 resistors or getting rid of cathode bypass caps from the stage.

Rick Davis said...

Yep, T-Bone, all true. Thanks so much for the info. I was referring only to the gain factor of about 40 percent.

Sometimes I think I get too arcane on the blog. I try to make it meaninful for the average player who is interested in amp tone but who is not a techie.

As the article says, players often replace premap tubes in amps out of habit/past experience/advice from others, etc. Most don't know or care about plate voltages. They care how stuff sounds. In this article I tried to demonstrate it.

Thanks again for your good comment.

exdmd said...

Rick, do you have an ETA for selling the VHT Special 6 with all your mods already done?

Blue Lip said...

Rick, I have tried to remove the back of my VHT Super Six to no avail. I have removed the six screws from the back panel but cannot get it to release from the cabinet. Are there some more screws that I need to remove such as the ones holding the corner cabinet protectors on? I am hesitant to get physical with the removal as I would not want to damage my new amp.
Helpful directions would be appreciated.

Rick Davis said...

Blue Lip- It was the same way with my Special 6 amp, and I have heard from other owners who had the same problem. The glue for the tolex covering seems to have extruded into the joint betweeen the amp and back panel during the assembly process.

I took a flat-bladed screwdriver and worked it under the back panel over the chassis. Yes, you will probably get some small scratches while prying that way, but it is the only way I could do it. Tne back panel did not break.

Good luck. I know it is a pain.

Exdmd- I don't think Bruce at Mission Amps is going to offer the modded amp in any kind of mass produced way. Too busy. Sorry.

Blue Lip said...

Rick, Thanks for the panel info. I have a flat paper folding knife that worked great for removal without leaving any scratches on the back of the power housing. Now, I just have to meticulously remove the staples and get rid of the screening.


Rick Davis said...

Steve, are you removing the back panel screen to make room for a bigger speaker?

LITTLE JOE said...

anybody swapping the speaker in this amp ? what did you put in ? I think most of the original stuff sounds really good. I put in a 12 AV7 Rca pre amp tube. i'm not really concerned with trying to make it louder.

Bill_Leeds said...

I am new to this... with regards the the tubes, is all that is necessary is swapping one tube for another, or are there other things that need to be done which would require additional parts and or soldering?

Thanks, and I really appreciated the well written text

Rick Davis said...

Bill- When swapping preamp tubes of the same family there is no need to solder anything, just plug-n-play. These tubes are all 9-pin mini tubes in the general 12AX7 family with varying levels of gain. Swapping the input tube for one with lower gain is very often the first thing a blues harp player does when he gets and new amp that was originally designed for guitar. There is TONS of info about this on the web, and it is easy to find.