What V4B tubes?

[MichelD]

An amp-building friend has offered to repair my 1974 V4B. Doing some research, I found two different tube formulas for the amp. Is there a significant tonal difference between using 7027A and 6L6GC tubes (valves) in the power section?



1. The Invalid Link Removed design used four 7027A valves for its power section and two 12AX7s – a 6K11 and a 12DW7 – for the gain stage



2. For tubes, four 6L6GC units run the power section, 12AX7 tubes are used for preamp, and a 12AU7 is used for the driver. .

 

[Wasnex]

An amp-building friend has offered to repair my 1974 V4B. Doing some research, I found two different tube formulas for the amp. Is there a significant tonal difference between using 7027A and 6L6GC tubes (valves) in the power section?



1. The Invalid Link Removed design used four 7027A valves for its power section and two 12AX7s – a 6K11 and a 12DW7 – for the gain stage



2. For tubes, four 6L6GC units run the power section, 12AX7 tubes are used for preamp, and a 12AU7 is used for the driver. .

I believe that era of V4 is pretty hard on the tubes and the original tube was 7027a. Not a lot of choices in current production 7027a, but Tung Sol has recently issued a version that is well reviewed and I believe @beans-on-toast has used/likes them in his vintage V4. Beans is a respected tech and knows a lot about Ampegs.

Tung Sol has also issued a well reviewed 7581a which is considered an upgrade to 6L6 with a higher plate dissipation rating. I believe true 7027a's have higher voltage limits than 6L6 or 7581A. I can't find a vintage Tun Sol 7027a data sheet, but the RCA 7027a was rated for 540V on the plate. Tung Sol is made by New Sensor and they are not very good about publishing data sheets for their current tubes. IMHO the old datasheets are totally irrelevant.

I am big fan of JJ but I don't believe the JJ 7027a holds up well in vintage V4s. The 1971 V4B schematic shows 545V is applied to the center tap of the output transformer. The voltage applied to the plates of the tubes would be very close to this value. The JJ 7027a datasheet shows a plate voltage limit of 500V.

There are some differences in the internal tube connections between 7027A, 6L6, and 7581A. Your tech may need to rewire the tube sockets in your amp to ensure compatibility across all tube types. Explanation: Sometimes amps used unused pins as a tie point to hold components. If the substitue tube has an internal connection on an affected pin, the component or tie point will need to be relocated.

Unless someone gives you a compelling reason to use something else, I would go with the Tung Sol 7027a...providing the old tubes are confirmed bad.

 

[beans-on-toast]

I like both the new Tung-Sol 7027A and the 7581A in the vintage V4B. I use the 7581A in my ‘74. When I bought the set, the 7027A reissues were not available yet. No modifications to the amp are required other than checking the bias.

 

[Wasnex]

I like both the new Tung-Sol 7027A and the 7581A in the vintage V4B. I use the 7581A in my ‘74. When I bought the set, the 7027A reissues were not available yet. No modifications to the ampare required other than checking the bias.

Do you notice any power or tonal differences?

 

[beans-on-toast]

Do you notice any power or tonal differences?

More clean headroom, tonally sounds very good.

I never tried to optimize power output nor did I record it. I dialing in the bias by ear and checked that it was within a safe limit. Tried a range of settings. I find that V4B’s sound good with a conservative bias setting and prefer the longer tube life that comes with it.