PP 2A3 amp rebuild

[David McGown]

Tom,

I ordered it, should arrive Monday. Thanks for the recommendation.

David

[tomp]

Another nice feature you will find is the ability to pre-program and save frequency sweeps.

Tom

[dberning]

Be careful of fixed bias on low-impedance triodes. I use servo autobias which works great, but it is more complicated. You might consider a partial cathode bias plus partial fixed bias as a compromise against throwing away too much power in the cathode resistor in full cathode bias. Problem with fixed bias is a small change in plate voltage supply causes a large change in plate current. Even regulating the plate supply is not a sure fix as the tubes themselves have a certain amount of instability because of gas.

[dberning]

Incidentally, I was surprised to find that my 845 p-p amp is Class A until clipping is reached. The early versions have a bias switch that allows the user to select what I termed "full class A", "partial" and "conserve". The conserve mode used a bias current of only around 9 mA per tube. I was surprised that even at this low bias current the definition of Class A is met: neither tube goes into cutoff for the entire cycle. I discovered this when I was looking at plate current with a current probe. Here is the reason. The plate impedance of these triodes, and I think it applies to other low-impedance triodes such as the 2A3 etc., is sufficiently low that as, say the top tube of a P-P pair is reducing current as its grid is driven more negative, its plate voltage is rising. The rising voltage offsets the grid's ability to turn the tube off until clipping stops the plate voltage from rising. Only then does it turn off completely.

[SoundMods]

Be careful of fixed bias on low-impedance triodes. I use servo autobias which works great, but it is more complicated. You might consider a partial cathode bias plus partial fixed bias as a compromise against throwing away too much power in the cathode resistor in full cathode bias. Problem with fixed bias is a small change in plate voltage supply causes a large change in plate current. Even regulating the plate supply is not a sure fix as the tubes themselves have a certain amount of instability because of gas.

This is an old G.E. trick that Jadis uses as well on their Defy 7 power amp.

[David McGown]

Tom,

Follow up on the function generator you recommended. Amazon actually cancelled and refunded my order since they couldn't deliver it. Probably didn't have it in stock. Anyway, looked at similar alternatives (seems that there are a couple of vendors with essentially the same device. The reviews were generally good, but there were a few issues with DOA or failures after a short period. The Siglient units looked good but pricey. Ended up buying a used HP/Agilent 33120A for $250. Feel a lot better about used quality professional test equipment than something new and cheap from China.

David

[tomp]

David:

Were you holding a gun at someone's head to get that price on the Agilent? Definitely a steal. The unit I got has been working great, but the construction is not what I would expect from Agilent. If it keeps performing like it does not, it will be great. If not, I'll have to start looking again. One thing I especially like about the one I have is that it is quite a bit smaller and lighter than the Agilent. As you get older you appreciate smaller stuff when you carry it out in the field.

Tom