JMP50 with GZ34

[Littlewyan]

Thought I'd start a new thread for modifying my JTM50. Anyway in case you hadn't guessed, its no longer a JTM50! The stock JTM50 with a GZ34 just didn't give me the tone I needed for my music. So I modified it bit by bit and basically ended up with a JMP50 with a GZ34 but lower filtering.

Its got the mids and its not quite got the clarity or bite/treble that I know these amps should have. At this point I'm suspecting either the filtering or the GZ34. I'm leaning more towards the filtering as I've heard David Bray's amps with very high gain running in valve rectifier mode and they sound great.

Current filtering set up is 32uF Mains, 32uF Screens, 32uF PI and 32uF+32uF Pre amp. As the mains and screens shares a Dual Cap Can I was wondering if changing it to a 50uF+50uF would make much difference. My 68 spec by the way is 820R/.68uF Bright Channel Cathode, .0022uF Coupling Cap, 5000pF Bright Cap, .68uF bypassing V2 cathode, 33k/500pf tonestack, .022uF PI Output Coupling caps and 47K Feedback Resistor on 8Ohm Tap.

[roberto]

I'd play a bit more with the NFB, as I usually like less NFB (higher values).
Install a 50k lin pot in series with the 47k and try to find a better value.

Then you can choose to keep the pot (maybe a 250k log) à la Rivera, or substitute the 47k with 100k or less.

[Stevem]

Bite and treble responce have nothing to do with filtering if that's what your implying!
Are you sure the amps circuit is not oscillating or phase canceling on you?
Feedback broadens the amps frequency responce, so lowering it will not get you more bite / high end, but differences in preamp stage B+ of even 25 volts can, as in more voltage, more high end.

[pdf64]

..Feedback broadens the amps frequency responce, so lowering it will not get you more bite / high end..

The gain of a open loop tube amp tends to tracks the load impedance, giving a bass and treble boost when driving a speaker, compared to a purely resistive load.
Series-shunt type NFB, as per Fender tube amps, acts to move the amplifier towards being an ideal voltage amplifier; this includes reducing its output impedance.
It's output voltage will tend to be constant over a wide frequency range.
Compared to its open loop performance, when set for similar nominal mid range output, the NFB amp's gain when driving a load whose inductive reactance is significant, eg a speaker >1kHz, will be lower.
Please try it out, the effect tends to be apparent, in that the open loop amp should have noticeably more treble.

[Littlewyan]

Its more the sharpness of the notes. So if I play an A Chord it sounds a bit mushy. I assumed the filtering would help as there is a lot of sag in the power supply at the moment and thought it may have been the issue. My amp is the same spec as the Metroamp 12000 series amps and they have a lot more bite. Its just so mushy at the moment.

I am using an attenuator and I do find the treble response increases as I lower the attenuation but I use the same attenuator with my TW Express and it works fine with that amp. I have plenty of bite at low volume with the Express!

I also used this attenuator with my previous Marshall 1987xl and didn't have this kind of mushiness. That amp had more bass and lower feedback! (.022uF Bright Channel Coupler, no .68uF Cap on V2 and 100K NFB on 8Ohm). I'm sure I could run that amp flat out as well with every control on 10. With this amp the clarity just goes out the window and the tone turns into a mess if I do that.

Edit: And when I say a mess I mean it sounds quite congested!

[Smokebreak]

I usually associate mushy/muddy/lack of clarity/congested with low mids. Raise the slope r.

[Littlewyan]

I'll measure it and make sure its 33K. It should be fine at that value with the .0022uF Bright Channel Coupling Cap. I remember trying the .0022uF Coupler on my 1987xl and finding it way too bright at any volume. Certainly not the case with this amp.

This is a 68 spec 100W by the way. Plenty of bite and it has exactly the same spec circuit as mine, minus the extra 2 EL34s, Solid State Rectifier and higher filtering.

https://www.youtube.com/watch?v=I49eCSgM1us

Its also been attenuated a lot so I guess that removes the attenuator from the equation.

[Littlewyan]

Just had a thought. I could try different EL34s. I'm using TAD EL34B-STRs at the moment and they didn't have much bite in my Express. I'll stick my JJ EL34 IIs in it quickly.

[norburybrook]

what about KT66's my JTM45 on the bright channel is very crisp and clear.

I have a standard values JTM 45 with KT66's . it's a very chimey amplifier.


this is through a Marshall 4x10 with celestions.


what are your bright cap values?


Marcus

[Littlewyan]

Volume Bright cap is 5000pF.

Mixer Bright Cap is 560pF.

Tonestack Treble Cap is 500pF.

I haven't had a chance to try the JJ EL34s but I noticed that the amp in the clip I posted is using the same valves. Marshall rebrand the Chinese EL34 which is the same as the TAD EL34B-STR . Pre Amp valves at the moment are a mixture, Tung Sol 12AX7, JJ ECC83S and JJ ECC803S (in V2 as it can take a higher heater/cathode voltage).

I measured the voltage sag in my amp tonight and the HT drops by 80V when I start playing. Never used a valve rectifier before, is that a standard voltage drop? I heard the old Mullard GZ34s didn't sag much at all.

[Smokebreak]

What are your EL34 and preamp voltages?

[pdf64]

what about KT66's my JTM45 on the bright channel is very crisp and clear

What make etc of KT66 are you using?

I measured the voltage sag in my amp tonight and the HT drops by 80V when I start playing

That's probably normal; you'll likely find that a significant part of the drop is due to the B+ winding voltage sagging, as with tube rectifiers, a degree of winding resistance is specified, in order to limit plate current.
Another cause of the VB+dc sagging under load is the reservoir cap being drained every half wave, thereby reducing the average Vdc.
So although the GZ34 doesn't contribute great to the sag, its requirement for highish winding resistance and smallish reservoir cap values can result in an overall saggy B+.

[norburybrook]

I'm using the Shuangen or whatever they're called, Chinese ones, they're supposed to be 'proper' KT66's not bastardised 6l6's some people make.


Marcus

[Littlewyan]

I haven't had a chance to mess with the amp yet. I'm hoping to get it out tomorrow night and play. One thing that came to mind is the EL34s are currently biased about 55% as I thought they sounded best set like that. However that was when I first built the amp as a JTM50. I guess they could sound best at a different bias now that I've changed the circuit.

So tomorrow night, measure voltages, try different bias settings and try different pre amp valves I think. I reckon I'll definitely need to up the reservoir cap to 50uF as the amp definitely falls apart when running everything on 10. I also wouldn't mind the bass being a bit tighter/stiffer and I believe increasing the size of that cap would help with that. Or wouldn't it?

Edit: I just thought, what I really meant in the first place is the amp is too soft. Not that it didn't have enough treble, but its so mushy that the treble gets a bit lost. There is no real sharpness to the notes.

[Tone Lover]

definitaly up the caps.
when i think of mushy bass and cathode caps have been ruled out. It falls to filtering.