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OK, I understand the purpose of the slot. Thanks Mark!
As long as the vent doesn't affect the sound of the HT2121...

I'll cut a new rear panel without the slot for my homemade cab, and test it to see if it sounds better.

What is the difference between a vent and a port? I can find links on the web about vent tuning and port tuning. So whether this slot is a vent or a port, is there any differences in the frequency response of these Hitone cabs, compared to the same cab without this slot? Dr Hiwatt, have you experien...

Well (excuse my bad english), as far as I can see on your last pic, there should be a resistor (1K5) in parallel with the 47n cap on the last turrets (at the right of the picture). Without that resistor, the bias of V3 is completely off. That may be why you have no sound. By the way, I've taken pict...

Beware, this post may offend some of you, hiwatt lovers, or even be considered as heretic by the Great Hiwatt Inquisitors! :lol: To make a long story short, I have a '78 DR504 that is proned to sound fizzy and farty. I have tried to see what's going wrong with my amp since 2005 and explored differen...

You did an excellent job on that 2x12 cabinet! Was it difficult to locate the Simms-Watts Fanes for it? Thank you! I grab the two speakers on ebay at a very fair price. They weren't far away from where I live, I've been lucky! :D Cool Binson Echorec, too. Hiwatt Bob has one that he brought to the 2...

No bad preamp tube, no cold solder joint, nothing. That's been many years since I periodicaly try to see what goes wrong in this damn amp. I've also tried all new components (resistors and caps, not only the power supply caps) without any improvment, so I've put back the original resistors and musta...

Hi Forgot to introduce myself! :oops: I am Laurent from France (that can't just explain why my english is so poor :oops: ), and I had the pleasure to read a lot of you on the vintageamp board. I have a DR504 from 1978 with which I am fighting for many many years now despite the fact I am a DIY'aholi...

Here are some news of my DR504. Well, I tried the conversion to mid 70's preamp and the loss of gain I was fearing is real. With that less gain, there is barely no "fizz". But when I plug a clean boost, the sound is fizzy again, as fizzy as the unmoded late 70's preamp when it is cranked! ...

Thanks Mikhailwatt! :wink: I've already seen your post and I was considering to do this conversion to mid 70's preamp as a next step if I couldn't find anything else. The problem is that I fear the loss of gain. :oops: But I suppose I can use a clean boost with the mid 70's preamp to achieve this ki...

I am a bit confused with this problem and the "De-fizz" correction. On my stock '79 DR504, when the preamp volume is crancked, applying a 300mV sinusoïde at the input, I read -8,6VDC (0VDC from 0% of volume to 50%) at the grid of V2, and in the meantime, the cathod of V2 goes from -1,7VDC ...