Music Man Tubes

[tdlunsfo]

I have a 2475-65 back on the bench after a full recap/retube 9 months ago. This is the tube PI model. The complaint is the classic rustling of leaves sound. I chased some carbon comp resistors but in the end I concluded two things, the noise is coming from one of the 9 month old JJ EL34 tubes and only in Hi mode. As soon as I swap to Lo mode it is dead quite. So... it appears I got infant mortality in an EL34 or I need to be more picky on what goes in this amp with 700+ plate volts. What tubes do you all put in your music mans?

[Jazz P Bass]

I am not a fan of JJ EL34 tubes.
I have had relatively good success with EH tubes.
(Then again, they all have a chance of failing)

[tdlunsfo]

Think I would have any better luck with 6CA7's?

[Jon Snell]

6CA7 is an EL34.
Courtesy of thetubestore.

EL34 vs 6CA7: two power tubes that can cause a lot of confusion when purchasing replacements, because they’re very similar and are, in fact, replacements for one another. They do, however have different internal designs, and they’ve changed slightly over the years.
The EL34 tube came first, and it was designed in the 1950s by the European company Mullard. Two EL34 vacuum tubes are capable of producing 90 watts of power under ideal circumstances. As a result, high-gain output made them very popular for use in guitar amplifiers, and they remain so today.

[tdlunsfo]

Yeah I knew they were close but not exactly the same. The schematic actually shows you can put in either. Some recommend the EL34L but its unclear to me the reliability benefits gained with that version.

[Jon Snell]

I steer clear of JJ. EH, made in USSR are excellent.

[g1]

Originally, I had thought the E34L had an extra little heatsink up top, and was rated at a higher wattage dissipation (30W)?
Looking at the new JJ datasheet, this is no longer the case. Otherwise I would have said it may be a good fit.

One thing I would try to make sure it's biased as cool as you can get away with. They call for 25mA with the tube PI, but at that kind of plate voltage that seems way too hot.
The non-tube PI models, they idle around 6mA, though that is cathode drive.

[tdlunsfo]

So this took an interesting turn. I called the store to inform them that we may had sold the customer some bad tubes and they informed me that the customer asked them at drop off if it would hurt the amp if it was left in standby all weekend. I have to be honest I have never pondered the idea. My first thought is it seems much more likely to have infant mortality on the tubes but then again what affect does baking tubes for 72 hours with no plate volts have?

[Enzo]

Can't imagine any particular problems.

[Ted]

If the tubes are 9 months old that hardly counts as infant mortality if the amp has been used at all. Those amps are extremely hard on tubes if they are biased too hot. What current are you running them at?

An EL34 which meets its spec should be good for 800V on the plate. (The Dynacord Gigant does this and gets nearly 200W from a quad of them in Class B )

The history of EL34s and 6CA7s is pretty convoluted. The original Mullard EL34 was a Pentode. The 6CA7 was an attempt to replicate it using a beam tetrode type structure. As tube sales shrank in the 70s, some manufacturers sold tubes labelled 6CA7/EL34 - I'm not sure whether this was a Pentode or a Tetrode.

[tdlunsfo]

I checked the bias with the new tubes and it was high. 750mv vs. the recommended 500mv at the cathode. Moved it down and double checked with my bias probe I get 23.5ma at 717v plate. Looking at my bias calc app this is right at 70%. So I'm going to run with this. Looks like the old tubes ran a little too hot plus the customer told me he played about an hour a week, so it appears these tubes did what they could do.