V4 PCB component repair/updating

[Hangman]

maybe you have a bad tube for channel 1? I've certainly had tubes where the heaters shorted to the cathode, and that made for a pronounced hum. that would also explain the interaction with the sensitivity rocker. since those are changing the resistance in series with the cathode bypass caps.

[beedoola]

maybe you have a bad tube for channel 1? I've certainly had tubes where the heaters shorted to the cathode, and that made for a pronounced hum. that would also explain the interaction with the sensitivity rocker. since those are changing the resistance in series with the cathode bypass caps.

Thanks Steve. I'll swap out V1 and see if that fixes the hum.

****Update****

Swapping out V1 stopped the extra hum.

[bax]

Good that you sorted it out.

Changing the tubes is always the easiest first thing to try. The problem is, sometimes the new tube can have an issue and hum just as bad as the first one. I once bought several tubes, all the same make and model, the first two that I swapped into V1 hummed just as bad. The third one resolved the issue. Two of the new tubes were bad.

I like to keep what I call a gold set of known good tubes that are only used for testing. It's kind of an extravagance but at least I know that I can count on them.

[Hangman]

good point.
I had had similar experience with the old sovtek made 7199. I bought about 6 of them and every single one of them hummed like mad. I ended up deciding to spend a bundle on NOS 7199s and some 6GH8A tubes that will work if you use an adaptor or rewire the socket.

The 7199 is such a great tube though, its a shame that nobody makes it. they were used all over in hi-fi amps, and ampeg used them in a lot of amps

Good that you sorted it out.

Changing the tubes is always the easiest first thing to try. The problem is, sometimes the new tube can have an issue and hum just as bad as the first one. I once bought several tubes, all the same make and model, the first two that I swapped into V1 hummed just as bad. The third one resolved the issue. Two of the new tubes were bad.

I like to keep what I call a gold set of known good tubes that are only used for testing. It's kind of an extravagance but at least I know that I can count on them.

[Hangman]

****Update****

Swapping out V1 stopped the extra hum.

Nice! I thought that might do the trick

[bax]

It's scary how the sock of NOS is running low. Tubes that used to be readily available just a few yours ago seem to be running low. Take the 6K11 for example, not too long ago it was around $8, the price has crept up to $30-$35. Lower prices can be found but the trend is less stock available and the prices higher. Substitutes are available that are more affordable.

I was recently looking for a tightly matched quad of NOS 7027A's. Pairs are a lot easier to source these days. I might need to modify my V4B bias circuit so that I can bias each pair separately. Fortunately the JJ's are available for those that want new production.

It makes you wonder where things will be in ten years.

The 7199 is such a great tube though, its a shame that nobody makes it. they were used all over in hi-fi amps, and ampeg used them in a lot of amps

[Hangman]

no doubt, I wish I had invested in a number of quads of 7027s about 5 years ago. the prices are so high now.

I worry about the state of tubes in 10 years. With the Winged C factory shutting down there is really just the new sensor factory in russia, the JJ factory in slovakia, and then a number of chinese factories in china.

the new sensor stuff is getting better, but its still nowhere near the quality of the winged C or NOS tubes. and with the current political situation in russia, the future is uncertain.

JJ is fine... they're not as good as winged Cs were but they are at least more reliable than most offerings from new sensor. However, they certainly do not seem eager to develop newer tubes, or improved versions of their current line
I give them credit for making the only acceptable 7591 right now, but Id love to see a 7199 from them.

It's scary how the sock of NOS is running low. Tubes that used to be readily available just a few yours ago seem to be running low. Take the 6K11 for example, not too long ago it was around $8, the price has crept up to $30-$35. Lower prices can be found but the trend is less stock available and the prices higher. Substitutes are available that are more affordable.

I was recently looking for a tightly matched quad of NOS 7027A's. Pairs are a lot easier to source these days. I might need to modify my V4B bias circuit so that I can bias each pair separately. Fortunately the JJ's are available for those that want new production.

It makes you wonder where things will be in ten years.

The 7199 is such a great tube though, its a shame that nobody makes it. they were used all over in hi-fi amps, and ampeg used them in a lot of amps

[beedoola]

In the ampeg, I thought the cooler bias happened to sound best--cooler bias like you have offered HUGE lows, and no mid-range mush. Big, loud, clean, but not harsh. I put back the stock value resistor, and have left it ever since. Note: with blackface style fenders and 6L6GCs, , I tend to like the bias closer to 50- 70% to get more mids I suppose.

I personally agree with this, too. I have a set of JJ 7027A's. These are definitely 30 watt tubes...unlike the original American made 7027A's which were actually 35 watt tubes. With a 528vdc at the plates, the bias on my VT-22 is set at 34ma's. It's a "cool" setting that to me it's win-win situation because the amp sounds right to me, and promotes longevity for the tubes.
BTW, here's my Adjustable Bias Mod:




I should have plenty of range with this setup - should I go insane and invest in a set of some NOS 7027A's...if they even exist! Oh yeah...there's not much of a margin for a shaky screwdriver, so I always use a insulated one!

Which resistor was replaced for the mod and what are the specs of the trim pot? - voltage tolerance?

[syscokid]

Which resistor was replaced for the mod and what are the specs of the trim pot? - voltage tolerance?

The resistor is at R49. The schematic states a value of 75K. What I found in my amp was a 1/4watt 68K...obviously not original! The trim pot has a voltage rating of 300v and 1/2-watt power rating.

Here's a link at the Mouser site for this trim pot:
http://www.mouser.com/ProductDetail/Bour...fYz1nVU%3d

[beedoola]

Is it necessary to use some kind of adhesive to keep the resistor/trimmer in place on the PCB? If so, would hot glue work, or is something else recommended?