Hi a popular input tube choice for the HD650 is the RCA 12AU7 clear top it is both widely available and inexpensive and a good place to start.
For the driver tube a good bang for your buck one would be something like a Chatham 6as7g. The Philco 6as7g I tried was also nice and from what I can gather where apparently manufactured by Amperex.
RCA 12au7 clear tops are easy to find on ebay and are often branded Conn just search ebay "12au7 clear top".
The Chathams turn up regularly and you might see some change from $20 for both tubes on ebay.
http://www.vacuumtubes.net/prices.htm have the Chathams 6as7g's for $20 each by the way
You will find that at the budget end having both 1 & 2 on your list will be challenging its not to hard to have either 1 or 2 on a budget, but not both at the same time so one or of them will be a slight compromise at the budget end.
Have a read of Nick Tams superb Multi 6080/6as7g post to get a handle on tube options for the Crack. It was original posted on the old BHF and in the move to the new website became truncated and a lot of the info was lost. But this link should show original post in full.
http://the-key.enix.org/~krystal/review-tube-bottlehead.html
« Last Edit: July 07, 2014, 11:18:45 AM by JamieMcC »
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