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Hello there, Does anyone out there have a step by step guide to converting a late 70's preamp to an early 70's preamp on a DR103?? I have a 1978 DR103 which needs to have a voltage double (+ bias pot) put in so that I can use some modern tubes. I put some Electro-Harmonix 6CA7EH in there yesterday and luckily I had a bias-rite meter on there because as soon as I switched the amp on the bias shot up to 75mA before I quickly turned the amp off. I can afford those indestructible NOS Mullards, so I will need to put in a voltage doubler/bias pot as well as increase the screen resistors to 1K 5watts (from the 100ohms that are in there now) At the same time I was thinking of rewiring the preamp section to an early seventies version, since I have heard that it has a better sound I know that the preamp doesn't have the Canadian mod but I can't tell what version of preamp is in there now. Below is a gut shot. If anyone knows where I can get instructions on the conversion, I would be appreciated.

Actually, yours does appear to have the Candian import gain mod... see the shielded cable running from one of the input jacks to V2?

It's really simple to convert V2 to the early '70s configuration. The gain modder already did some of the work for you, and the only additional parts needed are (2) 470k resistors and a terminal strip.

Here are Professor Huss' "de-mod" and "2-input de-fizz mod" pages for reference, you won't be performing all the steps: http://mhuss.com/Hiwatt/ca_mod.html
http://mhuss.com/Hiwatt/2_mod.html

First, disconnect the shielded cable (and the flying resistor, if there is one) from the input jack, then refer to the before/after pics and description in this thread: viewtopic.php?f=2&t=288

If you prefer the tone of the earlier classic Hylights, you'll really like this "correction." You will lose a lot of gain, but it's a good thing IMO.

Thanks.