Just bought a '77 DR504-Discovered gain and tone stack mods. Suggestions?

[loudstroud]

Hey Y'all - I'm new to posting here on VHC. I've been a fan of Hiwatt's forever, used to own a '71 Hiwatt DR103 and just recently purchased a '77 4-input DR504, the Hiwatt that has eluded me for a long time. Love it.

Thought this particular amp was an original untouched circuit, but discovered it had been slightly "adulterated" with a few minor mods to the input and tone stack.

What I found:
1. V1b cathode: the stock 2K2 resistor (in parallel w/ 47n cap) lowered to 1K5
2. V2a: A 1uf bypass cap added to the cathode (there was not a stock 47n bypass cap)
3. the 220pf cap in the tone stack (off of the treble pot leg) changed to a 1000pf.

The amp sounds really good, but a bit too bright. I don't mind the extra gain and clarity that I'm assuming is afforded by the lower value resistor on the input and the cathode bypass cap on V2a. I'm thinking the excess brightness might be caused in combination by the changed cap value in the tone stack and the cathode bypass cap on V2A.

So I'm planning on working backwards reversing the mods and start by putting the 220pf back in the tone stack.

So my question....Since the amp has already been messed with, are the any other bypass cap values (or input resistor) anyone might recommend trying?

Also while in here, I'm replacing the power supply electrolytics. Found the correct value cap cans thru ARS.

Any thoughts or suggestions will be most appreciated! I know Marshall's very well in terms of dialing in tone by minor circuit changes, but am new to Hiwatt's in that regard. For the most part, I'm a "if it ain't broke, don't fix it" guy and usually prefer just leaving them as is, so I am not opposed to simply returning this circuit back to completely stock values.

[mhuss]

Hello, and welcome.

Even stock HIWATTs are pretty bright, so it may be partly an expectation problem.

The 1000p will lower/widen the treble response, so 220p may seem brighter. This may have been an attempt to change the bright character of the amp.

1uF is large enough that it's probably just upping the gain and not changing the freq. response much.

The 1.5k may change the response a little, you'd have to swap it back to see what the sound difference is.

[loudstroud]

Thank you Mark! A pleasure to make your acquaintance here and appreciate your response. Exactly the input I had hoped for.

You are correct, Hiwatt's are brighter than I had perceived from the recordings I related them to. Which to a degree could also related to hearing them thru proper Hiwatt cab's with Fanes? Potentially accentuated brightness thru Marshall cab's. The speaker cab in my stash I've found most favorable for this DR504 is an early basketweave Park with 55hz Pulsonic cone G12m's. But the brightness more upper-mid voicing is a welcome discovery and preferred.

Both the 1000pf change and addition of 1uf bypass cap seem favorable mods that I'll live with longer. The change to the tone stack was more recently done by a tech who has had a lot of experience with these heads with pro players, so he might've just thrown it in there for good measure when servicing the amp (prior to me acquiring.)

The amp also has East German RFT EL34's, which I recall being inherently lighter on low end response. My friend bought two mid 70's DR504's together and sold me one of em. I have his other head here at the moment, which he say has more low end and why he sold me the other one. Once I finish swapping out the P.S. cap cans, it's a great opportunity to compare and decide on whether or not to reverse any of these mods. Agreed that the front input resistor value change wouldn't seem to make a significant difference.

[mhuss]

Yes, the old Fane speakers were better suited to HIWATT amps then, say, Vintage 30s.

[loudstroud]

Well I've replaced the power supply caps with ARS brand, who offer the closest to matching vintage values I was able to find. 500V as well which is better. Tightened up the tone a little, but honestly, otherwise not a significant tonal difference from what I recall. So apparently the original caps must've still been of relatively decent health for their age. Still glad I replaced to be 100% sure the P.S. mechanically is sound. Would much rather go to that trouble, rather than always have it in the back of my mind that the tranny's or any other part of the circuit might be under threat during heavy use or when pushed hard.

Have been A/B'ing with another '76 DR504 that belongs to a friend, which is dead stock.I dialed up great tones on both. Mine with the mod's sounds really good. But his sounds better and fuller.

With my amp, I like the additional openess and slight gain boost afforded by the 1uF bypass cap on the V2 cathode, but I'm thinking the 1000pf cap in place of the 220pf in the tone stack is changing the entire stack's wave form and interaction. It appears to affect the Mid control more than anything and has possibly rolled off some low end, to the point to when the Mid knob is anywhere above 1 o'clock, the Bass control becomes nonfunctional. With the Mid below 12noon the Bass becomes accentuated and the Bass knob works again. But still not as beefy as the dead stock DR504 I'm comparing to.

The true test was recording a guitar part last night and choosing the dead stock Hiwatt over mine. Both have RFT EL34's and mine even has a NOS Hammond branded Mullard ECC83 in V1, which was an improvement over the Philips ECG 12AX7A that had been in that position.

The first thing I'll try is replacing the 1000pf with a stock value 220pf and see what difference it makes.

[mhuss]

Definitely the first thing I would try.

[loudstroud]

Swapped out the 1000pf in the tone stack with the 220pf (NOS Mallory polystyrene similar to original) and while at it, put the V1 Brilliant channel cathode back to stock with a Mullard 47n and Iskra 2.2k. For the most part when A/B'ing to my friend's original stock, now they're really close, other than the more openess with the 1uF bypass cap on V2 cathode...

While in there I discovered another circuit change made on the Brilliant channel side of V1 previously overlooked. The plate resistor had been reduced to a 100k from its original 220k and the 1n coupling cap changed out for a 22n. Not sure how much difference the plate resistor value change is making....slightly more gain? Or had been adjusted for bias reasons due to the drop in the cathode resistor value?

But I feel like the 22n coupling cap is attributing to the Brilliant channel sounding not much different from the Normal channel, so plan on putting it back to stock. Found a proper mustard 1n on eBay that I'm waiting for in the mail.

End conclusion...Dave Reeve's Hiwatt circuit was well baked. These tone and gain mods made to mine were not improvements to my ears, other than maybe the cathode bypass cap on V2, which I'm also a fan of dropping in certain Marshall's.

[mhuss]

The 220k works better with the correct cathode resistor.