Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Vox and Hiwatt Discussion

Moderators: pompeiisneaks, Colossal

Post Reply
Travis_HY
Posts: 63
Joined: Thu Aug 13, 2009 3:18 am

Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Travis_HY »

Hey fellas,
Long time lurker, first time poster, long time solder sniffer and guitar tone enthusiast showing off a new build.



This is a 50 watt Hiwatt inspired build that fits in a JTM 45 sized box. It has a Heyboer Partridge clone power transformer and a Classictone Marshall output transformer for now. It's preamp is two channels like a typical Hiwatt, but I decided to utilize a ususally unused 12AX7 triode for the Jimmy Page/SAP preamp. This preamp is similar in tone to the Normal channel, but it allows the player to footswitch a variable volume cut control for a different tone. It works fairly well and offers two distinctively different tones for pedals or different guitars; one having more gain and bass, and the other for thinner rhythm work, or as a cleaner pedal platform. The footswitch was an XLR jack on Page's amp and I used a 1/4" in the front instead. The Brilliant channel has a larger coupling cap at .0047uf instead of the stock .001uf. There is a switch that toggles .047uf cathode bypass caps on the EQ and the first stage of the phase inverter for an alternative (and much brighter) Hiwatt spec.

I do have one question that perhaps someone could help me with: When you use bass heavy pedals with this amp, like a fuzz or an overdrive, and the bass is above noon; the bass is completely overwhelming. To the point where it sounds like it's oscillating and it's like a blocking distortion/ring modulator. Like a low end rumbling? Hard to describe. I tried a few things to remove this unpleasant sound, like reducing the .047uf capacitor in the tone stack that doesn't go to ground (I believe it's known as the "bass" cap) to a .022uf and lifting the "Deep" .047uf cap that goes to ground on the tone stack. The former seemed to help a bit and the latter made the amp much gainier, because the tone stack isn't being partially shorted to ground via that cap. I also changed the dual 470k mixer resistor network to dual 1Megs and that maybe helped a bit but the problem is pretty apparent still. Dialing back the Bass control helps, but it's still pretty intense with a pedal slamming it.

It almost sounds like a ring modulator only for those super low frequencies; which makes me think it might be related to the filter arrangement and it might need more filtering.

[IMG:800:800]http://i35.photobucket.com/albums/d196/ ... ijhwss.jpg[/img]
[IMG:800:600]http://i35.photobucket.com/albums/d196/ ... brmqm7.jpg[/img]

Thanks guys!
User avatar
Malcolm Irving
Posts: 139
Joined: Wed Jan 14, 2015 12:06 am
Location: Derbyshire, UK

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Malcolm Irving »

Travis_HY wrote: ...
It almost sounds like a ring modulator only for those super low frequencies; which makes me think it might be related to the filter arrangement and it might need more filtering. ...
The symptoms do sound like it might be on the edge of motor-boating. In which case I think the power supply filter caps are the first place to look.
Travis_HY
Posts: 63
Joined: Thu Aug 13, 2009 3:18 am

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Travis_HY »

Malcolm Irving wrote:
Travis_HY wrote: ...
It almost sounds like a ring modulator only for those super low frequencies; which makes me think it might be related to the filter arrangement and it might need more filtering. ...
The symptoms do sound like it might be on the edge of motor-boating. In which case I think the power supply filter caps are the first place to look.
I added another 22uf cap across the existing one and it certainly makes the low end more defined. I'm probably going to have to order a few more caps and add another stage of filtering; the last node is filtering 4 preamp stages so I figured that would be the best solution to just add another node that's a little lower voltage with adequate filtering.

Thanks for chiming in! :D
"Genius manifests itself. You got a hammer. You either build a Cathedral or you build a shithouse." - Carl Schroeder
JD0x0
Posts: 550
Joined: Tue Oct 09, 2007 2:19 am

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by JD0x0 »

Hiwatts were meant to have really heavy power filtering. Part of their tight sound, IMO.
It's true i've lost my marbles and i cant remember where i put them
Travis_HY
Posts: 63
Joined: Thu Aug 13, 2009 3:18 am

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Travis_HY »

JD0x0 wrote:Hiwatts were meant to have really heavy power filtering. Part of their tight sound, IMO.
I forgot to mention that the mains and screen filter supplies seem to hold together wonderfully in the low end. You can wind the amp up to arena volumes and it sounds full and mean like a Hiwatt should. As I did more experimenting (and I really should learn how to use a scope, because it would eliminate the guess work), I found that the first preamp stages were bleeding some unpleasant preamp signal through to another stage; probably the first tube of the phase inverter that shares its filtering node with the first stages.

I think that if you didn't use pedals, the lower filtering makes the amp a little more forgiving which I why I think DR went with the lower 16uf first stage filtering. The problem is that the lower preamp filtering can't be shared on more than 2 preamp stages in this particular amp it seems, which is probably due to my layout more than anything. However, I have heard a few Hiwatt clips online with very similar sonic qualities to what my amp is doing, so I don't feel like I'm too far off.

I looked at schematics on Hiwatt.org (Thanks Mark! What an amazing resource for Hiwatt nerds!!!), I see that the Jimmy Page preamp has higher filtering for the first two stages as well as another node and a fairly large dropping resistor to isolate the filtering for the first PI stage. I'm going to see if another stage of filtering might help. I don't think it will hurt; it might add some more stability to the preamp.

My ringing ears tell me the power amp seems good though! :D
"Genius manifests itself. You got a hammer. You either build a Cathedral or you build a shithouse." - Carl Schroeder
User avatar
Malcolm Irving
Posts: 139
Joined: Wed Jan 14, 2015 12:06 am
Location: Derbyshire, UK

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Malcolm Irving »

Travis_HY wrote: ... why I think DR went with the lower 16uf first stage filtering. ...
Also, electros were expensive in those days. I found a 1964 magazine which was offering 16uF at 450V for 2 shillings and 9 pence, and 32uF 450V for 3 shillings and 9 pence (retail). Those prices (allowing for inflation) translate to about $3 and $4 in today's money.

I'm thinking they tried a 16uF, found it was 'OK' and saved $1 on every amp! :D
Travis_HY
Posts: 63
Joined: Thu Aug 13, 2009 3:18 am

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Travis_HY »

Malcolm Irving wrote:
Travis_HY wrote: ... why I think DR went with the lower 16uf first stage filtering. ...
Also, electros were expensive in those days. I found a 1964 magazine which was offering 16uF at 450V for 2 shillings and 9 pence, and 32uF 450V for 3 shillings and 9 pence (retail). Those prices (allowing for inflation) translate to about $3 and $4 in today's money.

I'm thinking they tried a 16uF, found it was 'OK' and saved $1 on every amp! :D
Ha! That's funny. The bottom line is the bottom line.

Just spent a bunch of time getting this amp much closer. Added another stage of filtering and got the B+ adjusted closer to stock DR504 voltages.

Turns out a lot of the 'crashing' problem, if not most of the problem, was the lack of a input grid resistor after the Master Volume. There is a 22k series resistor between the wiper of the Treble pot and the input of the Master Volume. When I moved that resistor to the output of the Master going right into the grid of V2B it made the amp much more stable. I was having issues with oscillations with some controls zero'd. I thought it was was a lead-dress issue and I added a few shielded wires that certainly helped, but the oscillation persisted. I had been thinking there needed to be input grid resistors on both the input of the grid connecting the mixer network as well as the return of the EQ. Turns out just moving that EQ series resistor to the input grid of V2B cleans up those issues without affecting the tone of the amp.

This problem came up because I had also reworked the Jimmy Page preamp into a higher gain 800 style preamp; the cathode follower wasn't enough gain for me. The oscillation was super prominent when the added gain from the preamp was introduced. I'm glad I could get it sorted out. I'll post some sound clips when I get it dialed in 'mo betta.

Thanks for your interest!
"Genius manifests itself. You got a hammer. You either build a Cathedral or you build a shithouse." - Carl Schroeder
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Hiwatt DR504 w/ Jimmy Page & Brilliant Preamp

Post by Stevem »

Anything more than 2 stages of gain on one filter node of less than 50 uf is asking for the issue you have discribed!
You are right about the ring modulator type effect, as whenever you play loud enough or crank up the Bass enough, your getting your note (s) played modulated by power supply 120 cycle ripple.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Post Reply