SVT 6550 & 6146B Interchangeable?

slick_tu · #1 08-11-2010, 08:40 AM

While perusing eBay, I came across a '76 SVT with this tube chart pictured. Am I correct that certain mods need to be made in order to switch from 6146B tubes to 6550s?

KramerBassFan · #2 08-11-2010, 08:54 AM

A 1976 SVT is already using 6550's.

6146B's were used on the Earliest SVT's, and were phased out before very long.

How ever, Ampeg used the 6146B tube chart for a good time into the seventies, as you can see here.

rbonner · #3 08-11-2010, 08:56 AM

The tubes are different as in the 6146 has the HV plate cap on the top of the tube instead of coming in from a pin below. SO you have to change the wiring, plus the screen resistors need changing. There probably isn't even holes in the chassis for the plate leads to come through.

This amp was built for 6550's the tag inside the head is just a tube chart to cover a couple versions.

BOB

Seans · #4 08-11-2010, 09:00 AM

That's correct, mine was made in December 70 and uses 6550's, but the tube chart has no mention of 6550's just 6146B's. So they did revise the chart but kept the 6146B on there?..

anonymous02282011 · #5 08-11-2010, 09:02 AM

Do you any of you guys have both a 6146b version and 6550 version of the SVT?

I'm curious to know of any sonic differences, and how great those differences may be.

slick_tu · #6 08-11-2010, 09:08 AM

OK, I see what they meant by "or" on the chart; thanks for clearing that up guys.

Seans · #7 08-11-2010, 09:18 AM

Quote:

Originally Posted by adbass

Do you any of you guys have both a 6146b version and 6550 version of the SVT?

I'm curious to know of any sonic differences, and how great those differences may be.

JimmyM will let you know I'm sure.

rbonner · #8 08-11-2010, 09:42 AM

The 6146B was a precision, high tech and expensive transmitting tube even back in 1970. It was almost an honor and highly desireable in ham radio circles to get a 6146B for your transmitter than a 6146 or 6146A.

The 6550 was invented in around 1955 for Hi-Fi circles and quality ones of them were also spendy, but still not as much as a 6146B.

There's talk about instability in the SVT with 6146's. I truly doubt that and discount it as just purely talk. I seriously believe it was a financial reason they put 6550's in the amp. The tubes were cheaper and could be bought from both domestic and foreign sources. The 6146 was an RCA tube and Sylvania also built them, probably under license as a second source for the military.

As the military needs dwindled so did the $$$ to the tube manufacturers. This dealt the death blow to the cheap 6146B. Audiophiles will always need 6550's so their popularity continues.

The hot plate cap on top of the tube in a consumer product was also not the best idea. Rumor has it Keith Richards was killed several times during the 69 tour while drinking and drawing an arc off his plate caps with a pencil. Of course I started that rumor; because I need entertainment.

BOB

JimmyM · #9 08-11-2010, 09:43 AM

i have a 6146 version, and have played many a 6550 equipped svt, but i never did an ab of them in the same cab. i like them both quite well and the sound is similar. sedan dad on here has both, and he believes the 6146 svt is a little bit more powerful and warm, so i'll take his word for it.

anonymous02282011 · #10 08-11-2010, 09:50 AM

Quote:

Originally Posted by JimmyM

i have a 6146 version, and have played many a 6550 equipped svt, but i never did an ab of them in the same cab. i like them both quite well and the sound is similar. sedan dad on here has both, and he believes the 6146 svt is a little bit more powerful and warm, so i'll take his word for it.

I've got the 6146 version as well, but am always curious if the grass is greener on the 6550 side. I'd imagine they are more similar than different.

anonymous02282011 · #11 08-11-2010, 09:51 AM

Quote:

Originally Posted by rbonner

The 6146B was a precision, high tech and expensive transmitting tube even back in 1970. It was almost an honor and highly desireable in ham radio circles to get a 6146B for your transmitter than a 6146 or 6146A.

The 6550 was invented in around 1955 for Hi-Fi circles and quality ones of them were also spendy, but still not as much as a 6146B.

There's talk about instability in the SVT with 6146's. I truly doubt that and discount it as just purely talk. I seriously believe it was a financial reason they put 6550's in the amp. The tubes were cheaper and could be bought from both domestic and foreign sources. The 6146 was an RCA tube and Sylvania also built them, probably under license as a second source for the military.

As the military needs dwindled so did the $$$ to the tube manufacturers. This dealt the death blow to the cheap 6146B. Audiophiles will always need 6550's so their popularity continues.

The hot plate cap on top of the tube in a consumer product was also not the best idea. Rumor has it Keith Richards was killed several times during the 68 tour while drinking and drawing an arc off this plate caps with a pencil. Of course I started that rumor; because I need entertainment.

BOB

Nowadays NOS 6146b's are significantly cheaper than NOS 6550s.

KEITH LIVES!

coreyfyfe · #12 08-11-2010, 10:00 AM

Quote:

Originally Posted by Seans

That's correct, mine was made in December 70 and uses 6550's, but the tube chart has no mention of 6550's just 6146B's. So they did revise the chart but kept the 6146B on there?..

Yes. From what I've heard, they put out a factory recall to upgrade early SVTs to 6550s as well. I was looking at purchasing a mint 69 blueline with the hardwired speaker connector (and a beat to s**t squareback 8x10 that certainly wasn't original) which had been converted to 6550s in the 70s by ampeg and had the paper work to back it up. Too bad they were inflexible on the $4000 price tag....

anonymous02282011 · #13 08-11-2010, 10:01 AM

Quote:

Originally Posted by coreyfyfe

Yes. From what I've heard, they put out a factory recall to upgrade early SVTs to 6550s as well. I was looking at purchasing a mint 69 blueline with the hardwired speaker connector (and a beat to s**t squareback 8x10 that certainly wasn't original) which had been converted to 6550s in the 70s by ampeg and had the paper work to back it up. Too bad they were inflexible on the $4000 price tag....

Corey, if you're still looking for that rig I know a guy in our hood who's looking to sell . . for about half that price.

coreyfyfe · #14 08-11-2010, 10:01 AM

Quote:

Originally Posted by adbass

Nowadays NOS 6146b's are significantly cheaper than NOS 6550s.

Who wants 6146b's now? A bunch of hams? pfft.

coreyfyfe · #15 08-11-2010, 10:03 AM

Quote:

Originally Posted by adbass

Corey, if you're still looking for that rig I know a guy in our hood who's looking to sell . . for about half that price.

Ended up getting my own. Piece by piece. Have a 74 head/72 cab. The 69 is still sitting at empire loan in providence if you ever want to check it out. The dudes who run the music side of the business there are real cool and have some very nice vintage gear. Most of the stuff comes with a decent story (see the SVT above), hopefully most of it's not just bs

.

BassmanPaul · #16 08-11-2010, 10:20 AM

Quote:

Originally Posted by adbass

KEITH LIVES!

Now THAT"S certainly debatable!!! I'm of the opinion that he iS dead but his body is too pickled to realize it yet!

Paul

archieaa1 · #17 08-11-2010, 10:37 AM

The 6146 was used on only the very earliest SVTs. A 1976 SVT should not have 6146 tubes in it. From what I have been told they switched from 6146s to 6550s after the first run somewhere around mid 1970. The 6550 proved to be more reliable and less expensive. Most of the SVTs that had 6146s have been converted to 6550s. The conversion is pretty straight forward and isn't hard to find. It been part of "The Tube Amp Book" for many years. I've never seen an SVT with 6146 tubes in the wild so, I couldn't tell you if it sounds any different. The 6146 tube is used mainly for radio transmitters. While its a fairly common tube and is still in production in the east, its never been common for audio. In fact outside of the early SVT, I'm not aware of another audio amp that uses it. I had a Hamerlund 6 meter transmitter and Kenwood HF rig that used 6146 tubes.

Archie

Seans · #18 08-11-2010, 12:19 PM

Quote:

Originally Posted by rbonner

The 6146B was a precision, high tech and expensive transmitting tube even back in 1970. It was almost an honor and highly desireable in ham radio circles to get a 6146B for your transmitter than a 6146 or 6146A.

The 6550 was invented in around 1955 for Hi-Fi circles and quality ones of them were also spendy, but still not as much as a 6146B.

There's talk about instability in the SVT with 6146's. I truly doubt that and discount it as just purely talk. I seriously believe it was a financial reason they put 6550's in the amp. The tubes were cheaper and could be bought from both domestic and foreign sources. The 6146 was an RCA tube and Sylvania also built them, probably under license as a second source for the military.

As the military needs dwindled so did the $$$ to the tube manufacturers. This dealt the death blow to the cheap 6146B. Audiophiles will always need 6550's so their popularity continues.

The hot plate cap on top of the tube in a consumer product was also not the best idea. Rumor has it Keith Richards was killed several times during the 68 tour while drinking and drawing an arc off this plate caps with a pencil. Of course I started that rumor; because I need entertainment.

BOB

Excellent post Bob.

I would love to own a 69 with the 6146B's in, just because it was the original idea and all the components were designed around that tube, it's really supposed to have them in, I going to start looking, after all what could be better than owning one SVT

.

JimmyM · #19 08-11-2010, 12:53 PM

about this 6550 conversion...you can find many svt's with 6550's. you can only find a handful of 6146 svt's by comparison so no way i'd do it. not only that, but you can find 6146b's and 6146w's (mil spec 6146b) really cheap now. i paid like $25 a piece for them. i will just try to avoid touching the plate caps while it's plugged in...it's not hard since there's a grill on it

BbbyBld · #20 08-11-2010, 12:59 PM

There is an interesting SVT article in the last issue of Bass Gear Magazine with an interview of the designers. I recall them talking about the swap over to 6550s.

I've been on the lookout for a good 70's SVT I can afford for years.

I've always heard that it's not easy to find a working 6146 SVT that's all original. In other words they all burned up at one time and required tech work, usually with mods, to bring them back. This could be BS, but I've always avoided those in my search because of that. Also, there's no plate cap to worry about and 6550s are easier to find.