1968 vs 1969 Super Lead sound

[Tazin]

Based on everything you mentioned regarding both amps nothing really jumps out as a single reason for the two amps sounding that dis-similar. I would have to agree with Neil and George that the next logical step would be to swap the preamp and power sections between the two amps in order to hopefully narrow down what might be causing the difference.

[VelvetGeorge]

Send a sine in the front and measure the signal in AC volts on the presence side of the NF resistor.

Also, while you have the sine running you can measure after the bypass cap of each gain stage. Check at the treble pot pin 2 also. Good to see the signal level leaving the preamp.

Compare between amps, of course.

george

[shakti]

I just checked, and discovered that there *is* a possibly important difference between the two amps. The 68 has a Marstran choke, and the 69 has a M e r r e n choke. They are both supposed to be C1999 clones with a 3H rating, but from previous experimentation I have found that chokes can have a pretty profound impact on tone and feel, and in exactly the way that these two amps differ. I wrote a thread about it here:

http://forum.metroamp.com/viewtopic.php?f=3&t=39112

In that thread I wrote about comparing chokes in a JTM45/100 and a JMP50, and ended up really liking a Marstran in the JMP50 (where it has stayed). It imparted a slightly nasally, fat quality in the JMP50, increased the perceived gain and "rounded" the tone compared to a 5H 352-114 clone. That's much of the difference that I am hearing between my 68 (which is fat, nasally and feels gainy and round) and my 69 (which is stiffer, cleaner, tighter and faster).

It looks like some choke A/Bing is in order.

[Tazin]

Sounds plausible given the testimonials from yourself, Roe, and others in that thread. It will be interesting if the sound difference between the two amps is related to the choke(s). I won't have figured on too much difference between Chris's and Brian's product since I assumed both were based on original tear downs, and therefore they'd be using the same recipe.

[neikeel]

Sounds plausible given the testimonials from yourself, Roe, and others in that thread. It will be interesting if the sound difference between the two amps is related to the choke(s). I won't have figured on too much difference between Chris's and Brian's product since I assumed both were based on original tear downs, and therefore they'd be using the same recipe.

Nominally both the 354-114 and the C1999 are the same spec with 3H, 100mA rated with DR of about 100 or so ohms, does Chris have his at higher mA rating?

[VelvetGeorge]

Yeah, those should be pretty close in spec. The biggest tonal difference I can blame chokes for is going from a 3H type like these to a 20H type. Especially in a 100 watt.

george

[Tazin]

On one of my 100 watt amps I changed the choke from a Heyboer 5Hy (105 ohm) HTS-7262 to a Classictone 40-18058 3Hy 250mA and I didn't notice any real difference.

[shakti]

I don't know if Chris winds his with an increased current rating. I do think he did that with the one I ordered for a 45/100, and I ended up liking a Metroamp/Heyboer 352-114 clone better for that application. Now if this does have something to do with the choke, then my 69 with a M e r r e n choke was much closer to the real 69 I compared it with than my 68 with a Marstran choke is, even with same filtering specs.

In your case, Tazin, the low current rating of the 5H choke would mean that it drops in inductance when 4 x EL34s are drawing screen current, so it would probably behave fairly similar to a 3H choke with a higher current rating.
edit: I misread your post...don't know the current rating of the Heyboer...

In any case, this is just speculation so far. I will report back when I find the time to do some testing.

Does anyone know how much these original chokes varied in performance? I suppose variance in choke tolerances, current abilities and so on could possibly account for some of the variability in performance (sound and feel) of vintage amps with same specs.

[Tazin]

I believe the design specs on the Heyboer HTS-7262 are:
5Hy, 150mA
D.C.R: 105 ohms

The specs on the Classictone 40-18058 are:
3Hy, 250mA
D.C.R.: 112 ohms

[shakti]

I suppose 150mA is low enough that it would drop under heavy load. IIRC ROe said that 4 x EL34s and 3 ECC83s will draw about 140mA, give or take. I'm sure other factors will influence it, such as screen resistors, filter cap values etc.

[Roe]

I've tested with RFTs/Siemens biased hot. These tubes draw more screen current than most other el34s. I connect both chokes at one end (e.g. mains) and swap at the other end (e.g. screens) while the amp is on standby. After a lot of A/B testing I generally can tell a 3H choke apart from a 5h choke. I'm only using 3h and 20h now

[shakti]

OK, I rigged up to test the M e r r e n choke in the 68 amp. Somewhat to my frustration, it didn't alter anything much. I can't say for sure that there was no difference, I didn't set this up with a switch, but I know the sound differences between these two amps so well by now I would have been able to tell instantly if the choke was the deciding factor, and it is not. The 68 still has a slower attack, fatter and not as punchy. It crunches amazingly well, for some styles of playing this amp almost plays itself and I'm sure a lot of players would really, really love the way it plays and feels. And I do as well, but I still want that tighter, punchier sound sometimes.

I need to look elsewhere - either the preamp, or it could be something to do with the first power amp stage (rectifier and/or mains filter). I'm still thinking about how the 69 has essentially four 50uF filter caps in series/parallell. Perhaps that does something vs having two 100uF caps in series? Interestingly, one of the differences between a regular Hiwatt DR103 and the infamous Jimmy Page Custom 100 Hiwatt, is that the mains filter on the Jimmy Page model has four 100uF caps in series/parallell, vs two 200 uF in series for a regular DR103. Dave Reeves seems to have known what he was up to, so it's a little strange that he would go out of his way to use a different filter cap setup for that custom amp.

[shakti]

And another thought; the proximity of the OT and PT. They are much closer together on a 68 chassis. I thought I read something about the two influencing each other, but perhaps not in this way?

[shakti]

And finally, just an observation; the latest photos on Germino's site of his Headroom 100, shows that he is using 2 x 220uF ARS caps in series on the mains filter. This amp is his replica of a 68 SL, but he has still chosen to increase the filtering. If all else fails, I could easily add more filtering to the mains and still get a "2-in-1" amp via the dual voltage 3PDT switch; one mode with 460V and low filtering, and one mode with 490V and higher filtering.

[Tazin]

And the plot thickens. My gut feeling is that the filtering has something to do with the difference between the two amps....NOS verse new production caps.