Yes, I know the power supply voltages are higher than a true Express. Both the power and output transformers are Heyboer from Weber (I did not order them, they just sent them. LOL) and I cannot find any specs on them although when I measured the power transformer secondary, it was 377 VAC across each side. I used a dropping zener from Weber to drop the B+ by about 35 VDC. Bias is set at 40 mA cathode current and 452 VDC plate voltage (about 68%).
The amp is not harshly bright, it is just brighter than I would like. I currently have the controls set to P=6, B=5, M=8, T=3, and V=6. A Tele or Strat bridge is brighter than I want although I can bring things down with the guitar tone control.
Here is a photo of the guts. The amp is stable with all controls dimed. There are a few differences between this photo and the final layout, but nothing of great significance.
The speaker cab is a Crate that I got used at Guitar Center for $100. I have no idea what the speakers are. It could be that the speakers themselves are just bright. I have not had the chance to try it with another cab.
Here is a demo done by my guitar instructor. He had the tone controls of my Fender MIM Standard Strat (with Fender SCN pickups) rolled down a bit, especially the bridge. The bright switch does not need to be used, even with humbuckers.
The speaker cab is a Crate that I got used at Guitar Center for $100. I have no idea what the speakers are. It could be that the speakers themselves are just bright. I have not had the chance to try it with another cab.
Start there. The Express circuit is pretty picky when it comes to speakers, in my humblest opinion. I have seen Crates with the G12-35 speakers, which I remember liking, but I don't know how they mix with the Express...been a few days since I heard one.
The high B+ voltage will make the amp too bright, and too stiff. It won't allow for what I call "Dial a Crunch": using the guitar volume to control the overdrive in the amp.. An Express (and a Marshall, IMO) sound best ~400vdc B+ on the EL34's.
Hope this helps, some.
Keep experimenting. You'll find what works for you.
D
There are no stupid questions, just stupid people.......
I agree with Dobhill's assessment about speakers and operating voltages, but before you make changes to the circuit, you might try an instrument cable of at least 18-20' in length to roll off some highs at the input. What goes in, comes out louder. This is a Glen Kuykendall trick for moderating highs on an Express. Try rolling off your Tone control a little on your guitar. This will thicken the sound slightly without a loss of clarity depending on your guitar's electronics. Try rolling different preamp tubes. If you do decide to change the circuit, consider increasing the PI snubber cap (the 100pF cap between the plates) to 220-500pF. That WILL take high end off. You may or may not care for the effect but it is worth hearing it for yourself.
The basic amp configuration I followed was the one from Ron Worley's Express build guide.
I do hope to put some better speakers in the cab, but they are not in my budget right now. I doubt I can afford something like four higher end Celestions, but if you have any suggestions for something at a lower price point, say about $250 for a set of four, I would like to hear about them.
As far as the B+ goes, I am happy with the overall sound. If you did not listen to the demo, you should. The amp is very touch sensitive. Everything you hear in the demo is strictly done through pickup selection and guitar volume and tone control manipulation. The regular power transformer from Weber (which does offer the option of a lower voltage secondary, the Heyboer does not) is currently out of stock. That's why I used the zener voltage dropper to get the voltage down. It was up around 480 VDC. Any suggestions for a power transformer that will fit the footprint of the current one?
I am familiar with Glenn K's cable configuration. Right now I am running two 10' Planet Waves American Stage cables between the guitar and amp, but there is a buffered ABY switch (Radial Twin City Bones) that switches the guitar between two amps. I have the drag control on the switch set so that the highs are reduced slightly. I guess I could try a longer cable between the guitar and the switch. I have several 20' cables to choose from at home.
Tube wise, I am using JJ Gold Pin 12AX7s and JJ E34L power tubes. I initially had regular JJ 12AX7s in the amp but found that the gold pin versions have a smoother top end. They helped a lot. I am just a home player and can't justify the cost of NOS 12AX7s. The three gold pin JJs were less than the cost of one of the cheaper NOS 12AX7s and I know I can easily buy replacements if something happens.
I have considered upping the PI snubber cap to 250pF. I may just go ahead and try that.
Regarding adding the 0.001 uF cap to pin 6 of V2b, does it really go to ground or is it in parallel with the 100K plate resistor? Is there a value for the PI snubber cap that works best with this mod?
Just FYI, I have a BSEE which I received back when tubes were still part of the curriculum. I am only a few years from retirement and am considering repairing amps and maybe designing and selling amps to give me something to do (and keep from driving my wife crazy) and maybe earn a few dollars. I have been studying several tube amp design books to refresh my memory (I am a systems engineer for a contractor to one of the major US intelligence agencies. No, not NSA though I have worked for them in the past. LOL). I have built a 59 Bassman clone as well as made significant mods to a Fender Champion 600 (the Chinese version) and a Fender Blues Deluxe reissue, so I know my way pretty well around a tube amp.
bluesky636 wrote:
Regarding adding the 0.001 uF cap to pin 6 of V2b, does it really go to ground or is it in parallel with the 100K plate resistor?
The mod is really to ground. Although I think in parallel to the plate resistor works too. I'm kinda fuzzy on this though. Just recently read something like this in Merlin's preamp book. I think to an AC signal, B+ is as good as ground. You are just bypassing the AC from the plate resistor (for higher frequencies).
Anyway, you have more background than I in this regard. I can just say that .001uf from plate to ground was a common cure for folks who wanted to tame brightness.
Eminence makes a bazillion speakers to suit virtually every taste and most are in the $75.00 range. I have an Eminence Red White and Blues that would probably work great with an amp that is a tad too bright.
<i> "I've suffered for my music. Now it's your turn."</i>
Weber did the same thing to me with the PT switcheroo last year . I believe the java uses the WPt45, same as my plexi, and they sent the HY022798, probably the same one they sent you(should be on the side of the PT). I remember somehow finding the hy specs and it was around 740CT IIRC. That's a whole nother story...but point is the footprint was the same as the WPT45 . My Express runs 440V plates, and is THE contributing factor to the bit of harshness I have in that amp. I got it down to 410 with Zener string and it made a big difference, though I just wasn't comfortable with the Zener trick so I yanked it. Increasing the PI snubber will tame the high end, but blankets the tone a bit in my experience. Totally worth a shot though. Tube rolling in V1 has a massive effect, more than any other amp I've experienced. It took me a while to find one that wouldn't send the amp into sqealville, much less sound good. A new Sovtek actually sounded the best for me as far as new 12ax7s go. A JJ 5751 sounds pretty good too.
Have you tried the stock Express pot values?
bluesky636 wrote:
Regarding adding the 0.001 uF cap to pin 6 of V2b, does it really go to ground or is it in parallel with the 100K plate resistor?
The mod is really to ground. Although I think in parallel to the plate resistor works too. I'm kinda fuzzy on this though. Just recently read something like this in Merlin's preamp book. I think to an AC signal, B+ is as good as ground. You are just bypassing the AC from the plate resistor (for higher frequencies).
Anyway, you have more background than I in this regard. I can just say that .001uf from plate to ground was a common cure for folks who wanted to tame brightness.
Mike
Ok. Thanks.
I am by no means an expert on tubes amps, but I do spend a lot of time researching the amps I am interested in and have read many of the threads dealing with Express voicing in this forum.
Thanks. I have looked at the Weber speakers. They have a nice selection. I have an Eminence Cannibis Rex in my Blues Deluxe Reissue and like it a lot. Its kind of a middy speaker and may not work well in a closed back cab.
Smokebreak wrote:Weber did the same thing to me with the PT switcheroo last year . I believe the java uses the WPt45, same as my plexi, and they sent the HY022798, probably the same one they sent you(should be on the side of the PT). I remember somehow finding the hy specs and it was around 740CT IIRC. That's a whole nother story...but point is the footprint was the same as the WPT45 . My Express runs 440V plates, and is THE contributing factor to the bit of harshness I have in that amp. I got it down to 410 with Zener string and it made a big difference, though I just wasn't comfortable with the Zener trick so I yanked it. Increasing the PI snubber will tame the high end, but blankets the tone a bit in my experience. Totally worth a shot though. Tube rolling in V1 has a massive effect, more than any other amp I've experienced. It took me a while to find one that wouldn't send the amp into sqealville, much less sound good. A new Sovtek actually sounded the best for me as far as new 12ax7s go. A JJ 5751 sounds pretty good too.
Have you tried the stock Express pot values?
Yes, I saw your thread on the B+ issues with your amp. I received the same Heybour you did. I measured 377 VAC per side which would be 754 VAC. I used the Weber V-Dump Zener kit which brought the plate voltage down to around 750 VDC at my bias point. It sounds good and is very touch sensitive, just a little too bright.
As I mentioned, the JJ Gold Pin 12AX7s made a noticeable difference in noise and treble smoothness over the regular JJs, I would just like things a little smoother. I did try a NOS JAN Philips 5751 in V1, but felt it dropped the gain to much. I run NOS JAN GE 12AY7s in my 5F6A clone and BDRI for V1. I have no problems with squealing even with all controls dimed.
I initially raised the PI snubber from 50 pF to 100 pF and that helped a bit. I will probably try 250 pF.
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