This is my first post on this forum. Thank you for letting me in. Although I have been playing the guitar for quite a while and owned many amps over the years, I am a neophyte with regards to Hiwatt amplifiers. I'm sure I will learn a great deal with your kind help. I recently acquired a 1972 DR201 (the 4 x KT88s model) from the UK. I made the necessary AC plug conversion, repositioned the mains selector to 115V and changed the mains fuse to a 10A. I rebiased so that at 720 plate volts, the KT88s draw 35ma. Everything seems to be ok. Nice big open sound with an amazing break-up crunch. Being used to Marshalls, Fenders and Voxes, the voicing possibilities of my new amp are very different and almost "hifi" compared to the other ones. I currently plug it into a Marshall 1960TV 4x12 until I can find its soul mate. The only problem I am experiencing is that the mains fuse keeps blowing after about 10 minutes of playing. Can you suggest what might cause this? Any input will be much appreciated.
ThunderOne wrote:
Being used to Marshalls, Fenders and Voxes, the voicing possibilities of my new amp are very different and almost "hifi" compared to the other ones.
Welcome to the wonderful world of HIWATT! You hit the nail on the head !
ThunderOne wrote: I currently plug it into a Marshall 1960TV 4x12 until I can find its soul mate. The only problem I am experiencing is that the mains fuse keeps blowing after about 10 minutes of playing. Can you suggest what might cause this? Any input will be much appreciated.
I've been told that a mains fuse blowing can be attributable to a bad power tube, socket or connections. Although, in my experience, that usually causes the fuse to blow immediately. In any case, take a close look at your power section.
BTW, I've also got a 4x12TV cab that I really like. Not exactly the perfect Hiwatt-Fane match, but extremely nice with my 50 watt Marshall heads.
Thank you for your input, Dave! I will try a quartet of new KT88s I had waiting "just in case". Can you tell me why there are 2 trim pots on my amp, each seemingly hooked up to the 2 middle power tubes? I biased using the one on the right side when those pots are facing me (i.e.: amp upside down with front panel near me and power tubes in the rear part of the chassis). The one on the left didn't seem to do much. Could it be something like a balance control?
ThunderOne wrote:Can you tell me why there are 2 trim pots on my amp, each seemingly hooked up to the 2 middle power tubes? I biased using the one on the right side when those pots are facing me (i.e.: amp upside down with front panel near me and power tubes in the rear part of the chassis). The one on the left didn't seem to do much. Could it be something like a balance control?
Each trim pot controls bias for one pair of KT88's. If one isn't doing much, perhaps that's the weak link in the chain. Be sure to check bias on both inner and outer pairs.
OldSchoolDave wrote:Each trim pot controls bias for one pair of KT88's. If one isn't doing much, perhaps that's the weak link in the chain. Be sure to check bias on both inner and outer pairs.
Thank you for your quick response, Dave! I'll proceed at once. (Note to myself: the "left pot" controls the 2 outermost tubes and the "right pot" the 2 innermost ones. )
I have yet another question. I noticed something odd. Upon biasing the 2 innermost tubes (I used a Biasprobe installed on the 3rd tube from the left), I get positive values in ma and the trim pot is almost full clockwise to reach the 35ma target. But for the 2 outermost tubes (Biasprobe installed on the 1rst tube on the left side), I get negative values and even with the trim pot full counterclockwise, I cannot get lower than -37ma. Am I doing something wrong or is this something to be expected?
Dave, thanks again for your help and warm welcome. I just went ahead and flipped those toggles! Oohhh boy! She's sure is alive now! The old tubes got insanely hot before the newly installed set (Russian Gold Lions) and there was rattle in one of the bottles (yes, I know, not good! but hey, 3 of them were original Genalex Gold Lions so I figured I was clear for take-off...) Now, for the short time my spouse let me indulge, I'd say things are much better (no blown fuse (good thing, it's my last one), "normal" temperature and hyper%@[@]!!% sound!) I'll resume the test drive tomorrow!
I noticed a strange mod on my tone stack board. It definitely does not look stock. On the pic, you'll notice that on lugs position 11 and 14, there might have been some component that is now missing. Instead, the top lug no.11 is hard wired to bottom lug no.14 and bottom lug no.11 to top lug no.14. Why? What can this achieve? Is refurbishing to original specs in order? My instinctive answer is: YES! But the amp sounds great atm. Could it get even better??? I'll start researching the matter...
Ok Doc! Will do asap. It's been a hard day of shuffling yesterday and I'll let it rest a bit. But I think it's easy enough to see that two pieces of wire are soldered in an X-shape between the lugs in position 11 and 14 starting from the left.
the buss wires should run directly across the board. I am not sure what is up with them being crossed on your amp.
My guess is that there was a mod there that someone tried to un do, but it would be best to study the schematics and then know what is going on there.
!
. A new quad of JJ's and it's back to "choir of angels" form.
