OT question dual preamp a la Steve Luckey

[Auke Jolman]

My latest build is a 50w dual preamp a la Steve Luckey. I finished it today and it sounds pretty good. I have however a couple of questions. Is there a way to introduce some sort of level pot so the output of the plexi and the 2204 preamp sort of match?

The other question I have is that I'm using an OT with a 3.3K and a 4k primary winding. Wich one is best for 2 El34?

Also the base-pot doesn't do anything. I guess I have to check the wiring?

[sluckey]

Is there a way to introduce some sort of level pot so the output of the plexi and the 2204 preamp sort of match?

There are four volume controls in that preamp. You can't match levels with those?

I had envisioned setting the MSTR VOL to some mid position. Flip to 1987 position and use the two 1987 VOL to blend the Bright and Dark channels to produce the tone you want at some comfortable volume. Flip to 2204 position and use the 2204 VOL to match the sound level of the 1987 channel. Now use the MSTR to set the overall sound level.

Give that a try and let us know if that works for you.

[HotBluePlates]

The other question I have is that I'm using an OT with a 3.3K and a 4k primary winding. Wich one is best for 2 El34?

Either one is fine. Depending on EL34 voltages, you may get very slightly more clean power with the 3.3kΩ primary.

If you want to, you could try both. I'm not sure you'd hear much difference between them except near maximum output power. Both are safe.

[Auke Jolman]

Thanks for the response.

I made the primary winding of the OT switchable, you're right, I don't hear any difference.

As to the setting of the sound level, it's true that I can match the overall outputlevel with the gain knobs, but than de volume of the 2204 preamp has to be pretty low. Turning it up for a bit more crunch increaseses the overall volume of that channel quit a bit.

Can I change (up) the value of the mix resistor a bit?

[HotBluePlates]

The problem is the 2204 channel has 2x cascaded 12AX7 stages, while the Plexi channel has 2x parallel 12AX7 stages. The 2204 channel will always be louder for the same volume knob setting because there is more amplification happening.

The simple answer is turn the Plexi channel volume controls higher to match the level of the 2204 channel. You could add a volume pot to the 2204 channel between V2B and V3A to turn down its level to match the Plexi channel, but then both channels drive the V3 stages equally-hard. That would nullify much of the distortion-difference between channels when using the master volume, because V3 has the stages most likely to distort in the preamp.

You only notice there's a level difference because they're side-by-side in one box. I'd suggest you don't worry about "what number" each volume pot is set to, and just turn them until they sound right to you. This is also a feature in my mind, because you can quickly switch volume/distortion levels without an external boost pedal; if this is foot-switchable, you have a definite performance advantage.

Adding just a mixing resistor to the 2204 channel by itself will also make that channel darker once its big enough to actually make the signals the same-size. You could use a virtual-earth mixer to get equal levels without a tone change, but that would require another tube or a solid-state device. The active device is configured to have a feedback loop around it, and the sizes of the individual mix resistors compared to the feedback resistor around the active device sets the amplification of each mixed input; using this circuit, you could match the levels between channels.

However, the virtual-earth mixer still doesn't solve the basic fact that each channel has different levels by design because they have their gain stages arranged differently.

[Auke Jolman]

Thanks for the response.

I see what you mean. I had a channel switcher lying around and I build it in just now. I do indeed can use the 2204 preamp as a boost for soloing. I must do some tweaking thoug, because the 2204 preamp is a little on the bright side. Are there some general mods everybody does? I read the article that tonelizzard has on modding the preamp.

[HotBluePlates]

I must do some tweaking thoug, because the 2204 preamp is a little on the bright side. Are there some general mods everybody does? I read the article that tonelizzard has on modding the preamp.

It's bright by design, because lows with heavy distortion makes the overall sound muddy, and the amp is intended for band-use (where you'll have a bass player and maybe a keyboard player filling in the bottom of the band's sound). So what sounds nice and full when playing unaccompanied may be muddy and indistinct when playing with a band.

But looking at the schematic... do you see the 470pF and 0.001uF caps? These pass highs around voltage dividers and make the channel brighter. The 0.68uF cathode bypass cap for the 1st 2204 gain stage makes the channel brighter.

If you make the 0.68uF larger, you will get more low end from that gain stage. 1-2uF is often enough to sound full without being muddy.

If you completely remove the 470pF & 0.001uF caps, the amp will be less bright. Apparent gain will be somewhat reduced, and you might not want all the treble reduction. If you make any/all of those caps slightly larger values, you will roll in more low-mids or lows, while maintaining the amount of apparent gain. I can't recommend values, you'll have to use your ears to find the sound you like.

[Auke Jolman]

Today I made some changes to the amp. First of all I added a PPMV. I can dime it down and stil able to hit the PI a little more, which is nice when engaging the plexi preamp. Also the wife likes it 

To tame the highs in the 2204 preamp, I took out the both 500 pF capacitators. This also got rid of a lot of hiss, which was present when that preamp was engaged. I used Mica caps. Are there caps that donś introduce so much hiss?

All in all Iḿ pretty pleased with the amp. I have to play it a lot though ( ) to get the feel of the amp ....

[HotBluePlates]

To tame the highs in the 2204 preamp, I took out the both 500 pF capacitators. This also got rid of a lot of hiss, which was present when that preamp was engaged. I used Mica caps. Are there caps that donś introduce so much hiss?

The caps weren't hissing, it was the tubes and resistors.

Look at the schematic. The Volume pots are voltage dividers. The 470kΩ leading into the 2204's Volume control is kind of a "pre-voltage-divider" ahead of the volume control. The 2204 channel's series 470kΩ resistors to ground are a voltage divider. The Plexi channel's 470kΩ mixing resistors can be viewed as a voltage divider (though not exactly in the same way as a volume control).

The caps you removed bypassed highs only around all those voltage dividers. Hiss is broadband, but the bulk of it that you hear as annoying is higher-frequency. While the rest of the guitar signal was being attenuated at various points, those caps allowed highs, and hiss to pass at full-strength. You took them out, highs and hiss were attenuated along with the rest of the signal, and you heard it as a hiss-reduction.

If you wanted to bring back the highs without hiss, you have to attack the cause of the hiss. If you used carbon comp resistors, switch to metal film. Some noise is unavoidable with any type of resistor, and amplification makes it louder. Some tubes will hiss more than others, so that may be a place to check; the one which will have the most impact is the first gain stage of each channel (because its noise gets amplified by the rest of the amp).

Other high gain amps try to reduce hiss by throwing away the highest highs. That's the point of the 100pF cap from plate-to-cathode on the first stage of the 2204 channel. Mesa might put such caps on every stage; the problem is their amps often throw away highs and also cut out lows and leave a midrangey sound which is a signature sound but also one you can't get rid of with any setting of the tone controls.

Some amount of hiss or noise is unavoidable if you set all volume controls to maximum except the final master volume. Ideally, your master volume wouldn't be set so low, and when you have hiss you also have a very loud guitar signal covering up the hiss.

[Auke Jolman]

I'm using carbon film 1W resistors, except for the high voltages. For that I use metal oxide 2W small (high voltages to the preamp tubes) en 5W for the 10K dropping resistors. I do not have "special" preamp tubes for V1 en V3.

Is it better to use metal film resistors in regards to decreasing hiss all the way or is it enough to use them for the HV?

Further more does changing the leads to and from the pots with shielded wire make the amp make less hiss?

Although the amp has a low hum level, I'm curious about the grounding. The way i did it in the amp is:

• Input grounded directly to the chassis;
  All the preamps daisychained to each other at the 50 uF cap. The ground of the first cap connected to the buzzwire
  The buzzwire connects alle the grounds from the pots and is connected to  a PT bold;
  

CT HV and CT heater also to the PT bold, this is also true for each ground from the big filter caps and the ground from the bias voltage;

Is there a better way to do the grounding. I know there's a lot of information all around the web. In fact it's quit overwelming, so I would apreciate some feedback on this issue

[TubeGeek]

Check this out for grounding reference:


http://el34world.com/charts/grounds.htm

[Auke Jolman]

Thanks for the schematic.

I see that here the input is grounded to the buzzwire. I will try this and let you know if there's any difference.  I use plastic jacks for the speakers and input. Does the ground from the speaker jacks also goes to the PT bold ground?

Any thoughts on the hiss question anybody?

[Auke Jolman]

Did some tweaking in my lunchbrake...

I turned on the amp and forgot to plug in the guitar cable comming from my pedalboard. There was almost no hiss to be heard. I guess I have to redo my pedalboard instead of the amp. Previously I played through a 50W homemade plexi, there was some hiss but not to bothersome. The amplification factor of the 2204 preamp does the rest I think.

I also changed the grounding of the input jack to the buzzwire. I could not hear any difference.

I want to thank all of you for helping me get allong better.

If anybody has some suggestions as to lowering the hiss comming from my pedalboard, please don't hesitate to join in!

[EL34]

Check this out for grounding reference:
http://el34world.com/charts/grounds.htm

Wow, I forgot about this grounding chart on my site
That was done many years ago

Looks like it is in need of a Visio update

[EL34]

I updated the grounding info page
http://el34world.com/charts/grounds.htm

Here's the new Visio grounding diagram

[Willabe]

WOW, you guys do nice work with the Visio program!!!!!!



                  Brad     

[EL34]

Yes, Visio is pretty nice

[HotBluePlates]

I turned on the amp and forgot to plug in the guitar cable comming from my pedalboard. There was almost no hiss to be heard.

Yep, try plugging the guitar straight into the amp and see how much noise the amp is really contributing.

If anybody has some suggestions as to lowering the hiss comming from my pedalboard, please don't hesitate to join in!

We'd need to know a LOT more about your pedalboard setup, and especially the particular effects used. But you could probably try patching in one effect at a time to see which contributes the most noise. I would tend to suspect any multi-effect device (except high-end studio effects), a distortion/overdrive pedal, and especially any device which doesn't have a true-bypass setting when off (you could be hearing multiple opamp buffers on such pedals, each contributing a little noise).

[Auke Jolman]

Here's a listing of the stomboxes on my pedalboard:

Boss TU-3 => Boss CS-3 (modded) => Maxon OD808 => Voodoolab Superfuzz => Valbruch sapphire drive => MXR phase 45 => Boss TR-2 => Red Witch Lily boost => Boss RV-3 => Boss DD-7

I took my pedal board apart and build it up again piece py piece. I started with the DD-7. Although it's said to be true bypass, the guitar signal dropped a lot when I plugged it in as it did with the RV-3 and TR-2. The OD808 was the first what upped the level of hiss, only to be topped by the TU-3 when using the output-jack. When I used the bypass jack the hiss is completly gone.

At his point I decided to take a different aproach. I'v put a line-out in the amp and used my dust gattering Peavy 50W SS-amp as the amp to take care of everything but the overdrive/distortion pedals.

So my set up now is:

Boss TU-3(bypass jack) => Boss CS-3 (modded) => Voodoolab Superfuzz => Valbruch sapphire drive => amp

Line-out => MXR phase 45 => Boss TR-2 => Boss RV-3 => Boss DD-7 => SS amp


It's a bit of challenge to get the levels right, but I think I'll get there.

Is there an easier way to avoid the signal loss that was introduced by the Boss pedals, besides the obvious of bying new pedals?

I think I build een ABY-pedal to take the tuner out of the signal path, so I can tune up quietly.

[HotBluePlates]

Easy way? There's probably no easy way...

If you were a major rock star, the old answer was to get a technician to build a loop system for you. The idea was you had all your various effects always on, and the system switched various combinations of pedals for you by deciding where your guitar signal was routed. Think of a large and expensive buffer and switching controller, which then sent your guitar signal down various cables to your different effects pedals, and chose what order and which effects were used.

You could emulate some of that on a small scale through switching products, such as those sold by Lehle (and probably other, for less money). I don't know about EU pricing, but Lehle's products are very expensive in the U.S. The benefit of this approach is you will not be replacing you present effects with some from other makers.

Or you can replace the offending effects pedals with those having true bypass. Of course, you may still need a buffer or boost pedal ahead of your effects to help drive all the interconnecting cables. The thought here is if the pedal loads your signal down when set to the no-effect position, then the pedal includes some amount of circuitry between input and output jack, so it's not the same as the pedal not being present between the guitar and amp. True bypass is the same as no-pedal, except you may lose some of the extreme highs in your guitar signal due to the capacitance of all the interconnecting cables between guitar and amp.

There may be ways to modify a pedal that's introducing loss, or even an internal trimpot that sets the dry signal level. I'm sorry that I can't help with that, because I'm not well versed on a variety of pedal circuits, and know the internal features of only a small handful.

Regardless, it seems you know which pedals in your setup are most likely to introduce noise or signal loss, which means you're in a good position to deal directly with those issues. You can also evaluate whether it's a big enough problem for you to do anything at all (or if it's something you can live with).

[Auke Jolman]

There never ever is a easy way, is there?

After reading your response HotBluePlates, I realized you where right of course.

I left the Maxon OD808 off of my pedalboard because it introduced a lot of hiss. The rest is as I wrote in my previous post. The only thing I'm curious about is  what's the best place for my booster pedal before or after the Signal suckers.

[HotBluePlates]

Maybe before them, but you should try each way.

If the boost pedal is just a simple buffer, you would want your guitar to hit it first before anything else. But if it also has a boost function (or is a buffer with an additional switchable boost), it may interact with others pedals in an unusual way. The only thing I can recommend is to try every possible way.

But most of my playing experience has been guitar straight into amp; maybe someone else will more experience can offer a better suggestion.

[Mike_J]

It has been my experience that changing the plate resistors feeding the first two preamp tubes (four tubes in the case of the Sluckey Plexi/800 amp)to metal films reduces hiss considerably.  I use the Dale's and have no audible hiss in my amps with the metal films in those positions.  I never changed the plate resistors on my 5f6a Bassman amp.  It has AB carbon comp resistors and the amp has quite a bit of hiss.

Thanks
Mike