Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on August 22, 2021, 09:32:56 am
-
My most recent Plexi-ish build has prompted me to do another Marshall-style amp. I recently acquired a JTM chassis and panel set off eBay and will be getting the parts list ready and figuring out beforehand if there are mods to incorporate into the amp. After doing some research over the past couple of weeks, its obvious there are not only a lot of variations of the JTM-style circuit (e.g. tube vs. SS rectified), but also a lot of potential mods out there. While the mods might sound good, sometimes these look much better on paper and in reality the amp will stay in one particular setting/configuration 90% of the time.
So I wanted to throw some ideas out there to see if anyone has tried these and if so would you recommend them.
1. Switchable tube vs SS rectifier. I found a great thread on this forum (different amp) for setting up a standby switch as a 3-position switch to select between the two. It seems like a relatively straightforward mod. The JTM was originally tube rectified but later switched to SS.
2. Switchable tone stack values/"raw" switch. Switchable slope resistor values (33K and 56k) along with a 500pf or 250pf cap, and/or a tone stack bypass. Ideally this would be a mini-toggle and/or push-pull on the mid control.
3. Cascade or hot switch. Not sure if this is really necessary as I already have a 1987 and 2004 style amp. Wouldn't hurt to have the option but could introduce noise.
4. Here's a more involved one: KT66 and 6V6 operation. I got this idea from a Trinity amps Triwatt I built last year. It does a couple of things...when the amp is in 6V6 mode a Zener is inserted into the HV to drop voltage, and the bias range resistor value is changed. Of course this would require a re-bias but it definitely resulted in some different tones, at least on that Hiwatt-style circuit.
The amp will definitely have a Lar/Mar PPIMV and elevated heaters. It will probably end up having a Metro Zero-Loss FX loop (which I really like...its in the Plexi I built).
Thanks for any feedback, suggestions, etc. I appreciate the collective wisdom and experience in this forum.
-
One thing to consider... there seems to be a shortage on transformers right now. I had to wait months to get my power transformer. When they got the shipment in (Mojo) they only had 5 and they sold out almost immediately. So, if you don't have them already, I suggest ordering now.
-
Thanks for the heads up. Just looked at mojotone and they are pretty much sold out of everything. I know that the combination of magnetic components closing their doors and all the supply chain issues its been pretty bad. I got lucky back in March and was able to get a ClassicTone set for my 50 watt from Triode before the stock ran out.
Once I decide on KT66 vs EL34 I'll get the order in.
-
May be of interest...
https://el34world.com/Forum/index.php?topic=27827.0
-
there seems to be a shortage on
_(fill in your favorite item here)..........
-
ive had good luck getting hammond hardware from hawk.
-
Thanks for the info! I actually will be getting the PT, OT and head cab from a seller on this forum (thanks sluckey). Just need to source the choke which doesn't seem to be as hard to find as the PT and OT.
I've seen several iterations of choke for this amp...3H, 5, 7 and 20H. I've read that the higher values give a stiffer feel, but I just don't know a whole lot about that. Any suggestions and/or explanations are welcomed.
-
all the JTMs i'm seeing here use 20H: https://web.archive.org/web/20201112010824/https://drtube.com/en/library/schematics/69-marshall-schemas
i am not a marshall historian though, so others may chime in
-
I’ve used Hammond without issue. Tubedepot has them readily available.
https://tubedepot.com/products/hammond-194g-choke
-
all the JTMs i'm seeing here use 20H: https://web.archive.org/web/20201112010824/https://drtube.com/en/library/schematics/69-marshall-schemas
i am not a marshall historian though, so others may chime in
Yes it seems the schematics for the earlier iterations of the amp all have a 20H choke. However, when you look at the Metro build kit documentation, the kit references a 7H choke from Mercury Magnetics. Searching the Mercury Magnetics online parts for JTM chokes results in 2 choices: a 5H and a 7H. https://www.mercurymagnetics.com/products/?swoof=1&filter_make=marshall&filter_model=45-watt&filter_product-type=choke (https://www.mercurymagnetics.com/products/?swoof=1&filter_make=marshall&filter_model=45-watt&filter_product-type=choke)
On George M's store, he has a 3H choke for 64-66 spec amps. https://store.metropoulos.net/collections/transformers/products/metroamp-spec-drake-352-114-choke (https://store.metropoulos.net/collections/transformers/products/metroamp-spec-drake-352-114-choke)
I came upon this discussion from way back on the Metro forum about choke specs for a JTM, and it seems like they were all over the place. It seems like some of the guys posting know their stuff but its all new to me. Here's the link: http://forum.metropoulos.net/viewtopic.php?t=25243 (http://forum.metropoulos.net/viewtopic.php?t=25243).
So it sounds like the chokes are anywhere from 3H up to 20H, depending on year and maybe what was available to Jim Marshall at the time? Maybe a "middle of the road" 7H is the way to go (and maybe why Metro used this in their JTM kits back in the day). With all the other factors in play, is it discernible to the average player as to the differences in tone or feel among the various choke specs? I've only built one amp with a choke and have virtually no experience on how this component's spec will influence the sound.
Or maybe I'm just overthinking this and just get a 3H choke that are widely available and relatively inexpensive, as Blueboozer's link shows ($25).
-
My limited research agrees with PharmRock. 20H was spec’d on the schematics but a lot of the actual vintage amps seemed to get 3H chokes installed by Marshall. There’s some other forums where you can go deep down that rabbit hole. I don’t know enough to say how the # of henries would affect the tone to your liking or disliking. But it probably just boils down to preference and price more than authenticity or accuracy.
-
Or maybe I'm just overthinking this and just get a 3H choke that are widely available and relatively inexpensive, as Blueboozer's link shows ($25).
Good reading:
Chokes Explained (aikenamps.com) (https://www.aikenamps.com/index.php/chokes-explained)
-
The choke’s resistance and its current rating may be more significant than its inductance.
I like the high resistance 20H choke, as the response is a bit more saggy and screen grids are better protected from excessive dissipation.
-
Or maybe I'm just overthinking this and just get a 3H choke that are widely available and relatively inexpensive, as Blueboozer's link shows ($25).
Good reading:
Chokes Explained (aikenamps.com) (https://www.aikenamps.com/index.php/chokes-explained)
Thanks Silvergun, this was very helpful, as well as Pdf's post re: resistance and current.
In the Aikenamps link you sent, the following was mentioned:
"DC resistance: For a typical choke input supply, you need a choke with no more than 100-200 ohms or so DCR. A capacitor input supply typically might use a choke with a 250 ohm - 1K DCR. The higher the resistance, the more voltage drop and the poorer the regulation, but the cost will be lower.
Inductance:As for the inductance value, this depends on how much filtering you want. The inductance, in conjunction with the filter capacitance, forms a lowpass filter. The larger the inductor, the lower the cutoff frequency of the filter, and the better the rejection of the 120Hz (if full wave rectified) or 60Hz (if half wave rectified) AC component of the rectified DC. In general, the larger the better, within reason (larger inductances at low DC resistances mean larger chokes, which cost more money). Typically, 5-20 Henries is a good choice with the standard 32-50uF electrolytic capacitors. The inductance and capacitance values also determine the transient response of the supply, which means the tendency for the supply to overshoot or "ring" with damped oscillations whenever a current transient is applied (such as at startup or on a heavy current surge, such as a hard "E" chord at full power!)."
This statement I didn't quite understand: "The inductance and capacitance values also determine the transient response of the supply, which means the tendency for the supply to overshoot or "ring"....".
What exactly does this mean (overshoot or ring)? Along the same lines, how would the inductance value influence overshoot or ring? In otherwords, a higher inductance, the ______ the tendency for overshoot or ring.
Thanks for the help. Learning along the way.
-
Also interesting is Merlin B's discussion on chokes: http://www.valvewizard.co.uk/smoothing.html
About halfway down the page he suggests that Fender used chokes to drop less B+ and, thus get more clean. Since the Marshall was a copy of the 5F6-A, they used one, too. He seems to imply that an RC filter is a better (more economical) choice for a higher gain amp.
-
This statement I didn't quite understand: "The inductance and capacitance values also determine the transient response of the supply, which means the tendency for the supply to overshoot or "ring"....".
What exactly does this mean (overshoot or ring)? Along the same lines, how would the inductance value influence overshoot or ring? In otherwords, a higher inductance, the ______ the tendency for overshoot or ring..
More good stuff:
Overshoot and Ringing - diyAudio (https://www.diyaudio.com/forums/tubes-valves/182249-overshoot-ringing.html)
You might need to touch up on some serious fundamentals to digest all of this.
I'm chewing on some myself.
-
More, simplified, reading:
Tube Amplifiers Explained, Part 12: Power Supply (analogethos.com) (https://www.analogethos.com/post/power-supply)
I was able to grasp more today by reading some of Mr. Kuehnels' book (https://www.ampbooks.com/mobile/books/#!/Fundamentals-of-Guitar-Amplifier-System-Design/p/370629474/category=0)on System Design.
My takeaway from the last 2 days is that high gain designs require/demand more/better filtering due to the nature of overdrive and it's affect on the output stage.
A cleaner amp can be made to be more harmonically rich by allowing some ripple to remain in the power supply.
I just posted this yesterday in a different thread, mostly because this was a breakthrough for me:
Class AB Power Supply Ripple (ampbooks.com) (https://www.ampbooks.com/mobile/classic-circuits/class-AB-ripple/)
especially this: "Its modulation of the output appears to have a significant effect on the Bassman's sonic character."
-
get in the "habit";
every time you measure a power supply, measure both AC n DC, extra credit if you slap on a scope n look for transients
Keep putting in the book time, it'll pay off at play time
-
Thanks for all the suggestions. Just placed an Amazon order for Merlin's books. The power supply book is on back order but I'll keep scouring the web to gain a better understanding.
-
One of the things I was considering was a switchable SS/tube rectifier (similar to DR Z EMS). The PT I purchased has a 5V/2A secondary, which essentially restricts me to a GZ34/5AR4, which is what the original JTM came with. (I was thinking of using a 5U4 or GZ37 but those require 3A for the rectifier heaters). The GZ34 drops only ~10 volts (compared to the GZ37 and 5U4 both typically dropping around 40 volts) so I'm wondering if it would even be worth building in a switchable rectifier and instead just sticking with the GZ34 (since it has such little voltage drop).
Along those same lines, the specifications for a JJ GZ34 lists the typical operating conditions for a capacitor input first filter stage as 60uf, but this also happens to be the same as the limiting value (see attached data sheet). In a few kit schematics I've noticed that the first filter stage following the rectifier consists of two 32uf caps in parallel, whereas the original schematic is just one 32uf cap. I'm assuming that, due to +/- 20% variance, that a total of 64uf on the first stage is still within reason for typical operating conditions.
Which got me thinking...I'm guessing that the paralleled 32+32uf cap stiffens up the supply a little bit with a GZ34, which may accomplish (or get me in the neighborhood) of a SS rectifier. I may build it with a 32+32 cap can with just one 32uf connected at first, and then simply add a jumper to the other 32uf and see if there's much difference. If its noticeable, I may make the paralleled 32uf cap switchable.
Any thoughts/suggestions on any of the above is welcomed. Thanks!
-
Not as glamorous or convenient as a switched scenario, but you could get a plug in SS rectifier to try in the GZ34 socket to experiment first. Don’t get the one from AES. I bought two and they leak sand. Very weird.
https://tubedepot.com/products/solid-state-rectifier
-
Thanks Jordan...that seems like an easy and relatively cheap way to do this. I think Weber also makes these.
More research has led me to the Vox AC30CC schematic, which employs a "vintage/modern" smoothing switch...which switches in a parallel cap (22+22uf) on both the plate/OT CT and screen supply nodes.
https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf (https://el34world.com/charts/Schematics/files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf)
I suspect the 10K resistors across the switch terminals are there to reduce popping?
-
Not as glamorous or convenient as a switched scenario, but you could get a plug in SS rectifier to try in the GZ34 socket to experiment first. Don’t get the one from AES. I bought two and they leak sand. Very weird.
https://tubedepot.com/products/solid-state-rectifier
That really is strange -- the guy building them was most likely at the beach drinking margaritas while soldering. And just look at the schematic tucked away in the documentation -- you can build one for the cost of an octal plug and some diodes.
-
Not as glamorous or convenient as a switched scenario, but you could get a plug in SS rectifier to try in the GZ34 socket to experiment first. Don’t get the one from AES. I bought two and they leak sand. Very weird.
https://tubedepot.com/products/solid-state-rectifier
That really is strange -- the guy building them was most likely at the beach drinking margaritas while soldering. And just look at the schematic tucked away in the documentation -- you can build one for the cost of an octal plug and some diodes.
Takillya will impair your motor skills. :laugh: