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Old 26th April 2012, 02:08 AM   #51
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When drawing load lines for push pull amps you use 1/4 the plate to plate impedance. In this case 1.25K. This is what each tube sees on it's side of the output transformer to B+. Also, plate voltage can reach twice the B+ voltage when the tube on the other side is conducting hard.

To take a step away from the theoretical, I am posting X-Y plots of a 5881 in an actual guitar amp driven by a guitar. Sorry it's not an ultralinear output stage, but I don't expect one to be much different. The amp is a 5F6A re-issue from the 90's with GZ34 rectifier. Current is sampled by a 1 ohm resistor in series with the cathode. Plate voltage is measured by a 1000x probe connected to the scope's horizontal. (Don't try this at home unless you have the proper equipment.) In the first attachment, left most pic, I have sketched the plate curves and the 30W plate dissipation line. Note the screen voltage is such that plate current can reach over 400mA. The second pic shows the bias point. The 3rd pic is the load line with a 2 ohm resistive load. (The 5F6A drives four 10 inch 8 ohm speakers in parallel.) The second attachment shows what happends when playing guitar through the speakers. Note instantaneous plate dissipation can be well over 30 watts, plate voltage easily reaches 800V and can go negative several hundred volts.

This is the kind of stress that a guitar amp puts on it's output tubes.

Yes, I'm aware that a 5881 is only rated at 23 watt plate dissipation. These tubes are marked "5881 MADE IN USSR" re-branded "Ruby 6L6GCRI/5881 Made in Russia".
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Last edited by Loudthud; 26th April 2012 at 02:15 AM.
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Old 27th April 2012, 01:21 AM   #52
nazaroo is offline nazaroo  Canada
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Thank God that Fender makes crappy amps!

Turning to the physical construction problem,
I have solved this by the simple expedient of buying a piece of crap.

The Fender Pro 185 is admittedly a loud amp.
Unfortunately, its 'sound' is crudely synthesized by transistor/op-amp technology.

Typically, these amps (like all amps) develop problems like crunchy, noisey volume/tone pots, and crackly cutting-out symptoms, which are notoriously difficult to hunt down and/or permanently fix.

So, such amps have the double-strike that they are both NOT TUBE AMPS, and they have as many or more problems as tube amps.

As a result, their resale value is similar to that of an old foreign car, which offers expensive repair bills and little else.

Yet, for the savvy amp-builder, these boxes have one redeeming feature:
They are about the same in construction as a good Fender Twin Reverb (Tube version).

Obviously, the speakers, having to handle the same power, are the same, and so is the cabinet. To buy these parts individually would be cost-prohibitive, and to build a cabinet, while a worthwhile project on its own for a woodworker, is as expensive as just cannibalizing one.

So I picked up such a crappy amp used, for under $200.
Speaker and Cabinet problem solved:

Click the image to open in full size.

Click the image to open in full size.

These speakers are just about the best I could get,
because they are even better than typical Marshalls.
They are designed for abuse, and don't require mounting
in a sealed cabinet, but can handle pretty severe excursions,
and a lot of wattage.


The equivalent 140 watt Fender cabinet without amp would run about $500 new!

But the additional bonus is of course a virtually clean chassis,
with all the mounting parts and re-useable hardware,
e.g., A.C. cable, On switch, silkscreened frontplate etc. etc.

Click the image to open in full size.

This is another good reason not to sneer at crappy old amps,
that would cost an arm and a leg to fix:
They are natural part-sources.

In fact,
here is the 140 watt transistor amp , complete with power tranny,
which I will sell as is for parts to repair another amp,
no reasonable offer refused! (Buyer pays shipping).

Click the image to open in full size.

I may have to cut back or rebend/fold the chassis,
but that is a small price to pay for securing a solid,
practical platform for my amp project.

This was a case of money well spent on this project.
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Old 27th April 2012, 05:56 AM   #53
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Loudthud View Post
When drawing load lines for push pull amps you use 1/4 the plate to plate impedance. In this case 1.25K. This is what each tube sees on it's side of the output transformer to B+. Also, plate voltage can reach twice the B+ voltage when the tube on the other side is conducting hard.

To take a step away from the theoretical, I am posting X-Y plots of a 5881 in an actual guitar amp driven by a guitar. Sorry it's not an ultralinear output stage, but I don't expect one to be much different. The amp is a 5F6A re-issue from the 90's with GZ34 rectifier. Current is sampled by a 1 ohm resistor in series with the cathode. Plate voltage is measured by a 1000x probe connected to the scope's horizontal. (Don't try this at home unless you have the proper equipment.) In the first attachment, left most pic, I have sketched the plate curves and the 30W plate dissipation line. Note the screen voltage is such that plate current can reach over 400mA. The second pic shows the bias point. The 3rd pic is the load line with a 2 ohm resistive load. (The 5F6A drives four 10 inch 8 ohm speakers in parallel.) The second attachment shows what happends when playing guitar through the speakers. Note instantaneous plate dissipation can be well over 30 watts, plate voltage easily reaches 800V and can go negative several hundred volts.

This is the kind of stress that a guitar amp puts on it's output tubes.

Yes, I'm aware that a 5881 is only rated at 23 watt plate dissipation. These tubes are marked "5881 MADE IN USSR" re-branded "Ruby 6L6GCRI/5881 Made in Russia".
This is great stuff!

I love the wild and woolly voltage/current excursions in your photos. Looks like a reflection of the reactance/phase effects of the speaker-load, and the lack of a stabilizing high resistance in series with the tube.
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Old 27th April 2012, 06:11 AM   #54
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Loudthud View Post
When drawing load lines for push pull amps you use 1/4 the plate to plate impedance. In this case 1.25K. This is what each tube sees on it's side of the output transformer to B+. Also, plate voltage can reach twice the B+ voltage when the tube on the other side is conducting hard.
.
The point about the actual impedance seen by one tube on one side is a great note.

Also, the fact that the plate-voltage can swing as high as 2 x B+
is a real warning that shouldn't be ignored.

I was actually being conservative in my concerns about voltage and internal tube-arcing.
Although the published ratings for NOS tubes are usually design-center values (with swings outside the max expected),
two things cause additional caution:

(1) modern tubes don't act like NOS tubes.

(2) design-center maxs don't have in view guitar-amp overloading, high extended current periods, and full sine-wave RMS signals on input grids.

We see a trend in many newer amp designs of lower base plate voltages (HV B+ design choices), and a return to tube-rectification and even PS choke mods.
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Old 27th April 2012, 11:17 AM   #55
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Quote:
Originally Posted by nazaroo View Post
The point about the actual impedance seen by one tube on one side is a great note.

Also, the fact that the plate-voltage can swing as high as 2 x B+
is a real warning that shouldn't be ignored.

I was actually being conservative in my concerns about voltage and internal tube-arcing.
Although the published ratings for NOS tubes are usually design-center values (with swings outside the max expected),
two things cause additional caution:

(1) modern tubes don't act like NOS tubes.

(2) design-center maxs don't have in view guitar-amp overloading, high extended current periods, and full sine-wave RMS signals on input grids.

We see a trend in many newer amp designs of lower base plate voltages (HV B+ design choices), and a return to tube-rectification and even PS choke mods.
How much are they derating the new amp voltages as compared to old? Do they bump up the current to get the same wattage amp or not?
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Old 27th April 2012, 12:03 PM   #56
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Quote:
Originally Posted by nazaroo View Post
Thank God that Fender makes crappy amps!
Maybe their newer ones, but that's already been debated ad nauseum.

Quote:
Originally Posted by nazaroo View Post
This is another good reason not to sneer at crappy old amps,
that would cost an arm and a leg to fix:
They are natural part-sources.

This was a case of money well spent on this project.
This is an great point and a great tip! I'm going to start looking for a 1x12 combo to use for my next project.
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Old 27th April 2012, 12:18 PM   #57
nazaroo is offline nazaroo  Canada
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Originally Posted by Printer2 View Post
How much are they derating the new amp voltages as compared to old? Do they bump up the current to get the same wattage amp or not?
I think there is less wattage too, but nobody will notice,
because the difference between 70 watts and 100 watts is less than 1 db!
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Old 27th April 2012, 09:44 PM   #58
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Quote:
So I picked up such a crappy amp used, for under $200. Speaker and Cabinet problem solved
I hit the local (and some not so local) hamfests to sell my junk, and shop for rare treasures. I look for dead guitar amps for cabinet reuse. The picture below shows the haul I brought back from the Orlando hamfest in February. I was looking for tubes when I got a phone call about a guy selling a Marshall 4 X 12 cabinet loaded with Celestions for $100. I ran to the location to find a friend buying it. The seller had several dead guitar amps that had been tweaked to death by an "expert". Some were missing parts. I purchased them ALL for $25 total.

I traded away the reverb unit at another hamfest. Got a Stratocaster in that deal. I have often found and sold audio and guitar equipment at hamfests. The interest is low, so are the prices. Rare pieces have high prices and don't sell. There was a Gibson SG at Orlando with a $600 price. That was a good price for that guitar, but people don't go to hamfests to spend $600 on a guitar.

I dragged them home and autopsied them all.

There was a Fender G-DEC (Guitar Digital Entertainment Center). I was curious enough about it to fix it. It is like the guitar equivalent of a Karaoke machine. It is unique enough as a TOY to keep me interested for now. I will sell it when it loses my interest.

The other three have been judged to be certified cabinet donors. The speaker in the Crate might be a keeper. The Fender Frontman speaker didn't like being fed 125 watts of tube amp scream and it has been permanently silenced!

I haven't said anything about the amp in this thread since the OP obviously has strong opinions on the subject, but here is my $.02. I have been making tube guitar amps since the 60's. Back then I used parts from old TV sets to make what I could. I can now build whatever I want, and will often breadboard something just to see what it sounds like.

It has been said that UL doesn't work for guitar amps. I tend not to listen to what others say, so I have tried it. I tried SE UL, and P-P UL. UL just doesn't sound right when the output stages are pushed into clipping. I have tried it a few times and never got anything that I liked, but your tastes may vary.

About 2 years ago I decided to build a "HiFi guitar amp". I used one of my HiFi amp boards (an SSE wired for P-P) and designed a preamp that had enough gain for a full metal racket if desired, but could be clean enough that the whole amp could do 50 watts at 2% distortion. I used two Hawthorne Silver Iris HiFi drivers because I had them and they can eat 100 watts each if needed. The amp sounded good and would be great for a keyboard, or acoustic guitar, maybe even an ES335 playing rythym. I however became bored with it after a few months and took it apart.

I am now building a semi holow body guitar, so I may revisit the HiFi guitar amp again, who knows.
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Old 27th April 2012, 10:16 PM   #59
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by tubelab.com View Post
I hit the local (and some not so local) hamfests to sell my junk, and shop for rare treasures. I look for dead guitar amps for cabinet reuse. The picture below shows the haul I brought back from the Orlando hamfest in February.
Great haul!
Nice score, even if you didn't get the 4x12.
At least its in the family so to speak.



Quote:

I traded away the reverb unit at another hamfest. Got a Stratocaster in that deal. I have often found and sold audio and guitar equipment at hamfests. The interest is low, so are the prices. Rare pieces have high prices and don't sell. There was a Gibson SG at Orlando with a $600 price. That was a good price for that guitar, but people don't go to hamfests to spend $600 on a guitar.
Sounds just about right.
I find that too. If I go to a non-musical pawn shop,
sometimes they underprice.

Quote:
"I dragged them home and autopsied them all. "
I love this quote!
You make it sound as fun as it must have been.

Quote:

The other three have been judged to be certified cabinet donors. The speaker in the Crate might be a keeper. The Fender Frontman speaker didn't like being fed 125 watts of tube amp scream and it has been permanently silenced!
The Crate ought to be good.
I'm not surprised the Fender speaker died.
A brand name ain't gonna make up for lack of an adequate wattage rating.


Quote:
I haven't said anything about the amp in this thread since the OP obviously has strong opinions on the subject, but here is my $.02. I have been making tube guitar amps since the 60's. Back then I used parts from old TV sets to make what I could. I can now build whatever I want, and will often breadboard something just to see what it sounds like.

It has been said that UL doesn't work for guitar amps. I tend not to listen to what others say, so I have tried it. I tried SE UL, and P-P UL. UL just doesn't sound right when the output stages are pushed into clipping. I have tried it a few times and never got anything that I liked, but your tastes may vary.
Yeah I was actually expecting a lot more flack for even mentioning UL modes.

I think you are right that untweaked UL ain't gonna be anything special guitar-amp wise.

But I had in mind this:

(1) UL gets you closer to triode than pentode mode, without losing too much power output.

(2) The type of distortion some crave coming from the the power-section seems to be based on a handful of independent factors:

a) OT saturation and impedance

b) the effects of screen-current in fixed-bias configurations, and the choice of resistors.

c) tube-type and frequency response under overdrive.

d) blocking distortion from the driver section just previous.

In my mind, these various desirable/undesirable effects are either independent of UL mode topology, or else can be brought into it or eliminated by design.

(3) UL simplifies PS design.

So I'm not ready to write off UL mode for guitar amps.

Quote:
About 2 years ago I decided to build a "HiFi guitar amp". I used one of my HiFi amp boards (an SSE wired for P-P) and designed a preamp that had enough gain for a full metal racket if desired, but could be clean enough that the whole amp could do 50 watts at 2% distortion. I used two Hawthorne Silver Iris HiFi drivers because I had them and they can eat 100 watts each if needed. The amp sounded good and would be great for a keyboard, or acoustic guitar, maybe even an ES335 playing rythym. I however became bored with it after a few months and took it apart.

I am now building a semi holow body guitar, so I may revisit the HiFi guitar amp again, who knows.
I hear you on this.
There is no 'magic bullet' design.
And even favorites come and go with mood and developments.
A change is as good as a rest.
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Old 27th April 2012, 10:36 PM   #60
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Quote:
Originally Posted by nazaroo View Post
I think there is less wattage too, but nobody will notice,
because the difference between 70 watts and 100 watts is less than 1 db!
Any amps in particular?

Quote:
(1) UL gets you closer to triode than pentode mode, without losing too much power output.
Most think triode mode is too bland for guitar, that is unless you like clean.

Quote:
(2) The type of distortion some crave coming from the the power-section seems to be based on a handful of independent factors:

a) OT saturation and impedance
One of the better upgrades done by guitar players is to beef up the OT if it is too wimpy. Never heard anyone say they would rather go back to the stoctk transformer once they upgraded.

Quote:
d) blocking distortion from the driver section just previous.
Same seems to go with blocking distortion. It does not seem to be desirable.
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