Channels, Speakers output and SS Rectifier diodes - questions

[uki]

Hey guys

To build a tube amp is a very good experience, I'm building my 1st amp,  although my electronic knowledge is limited I did learn a lot things by just doing it, still I have many questions and lots of things to learn, I'm enjoying it tons.

Here my questions
 
1- Channels
  a- One channel alone will do the total output of the amp?
  b -Linking channels. In Fender amps with 2 channels like the Twins, when linked, do they add up to the signal before getting to the power tubes, by linking the channels does it produce more gain?

2- In the amp when there are speaker socket and external speaker socket, lets use as example the 6G8 something I'm familiar with, as far as I understand thethe speaker sockets are in parallel yes? So it is for 4 Ohms load, and there is a 4 Omhs 2x12 cab connected to the speaker socket, what can be connected to the external speaker socket, another 4 ohms cab? But if the sockets are in parallel wouldn't it make those two cabs a 2 Ohms load total?

3- Rectifier and diodes, In my building of a 6G8 the rectifier I build there are 7 diodes, in the schematic there is no mention about what value they are, so I followed my instincts and what did make sense, and build it with the 1N4007/1000V, 1 amp rating but then reading this topic: http://el34world.com/Forum/index.php?topic=19174.0
I found this post:

> the Hoffman rectifiers appear to have 3 diodes in series

That's copied from a 1961 design (likely the standalone reverb), from days when 200V diodes were about the best we could get. The 3-stack gives nearly 600V rating.

Since today a 1,000V diode is as cheap as a 50V diode, we would just use one diode.

So it leaves me with the impression that something maybe incorrect in my rectifier. Do i have too much diode value there? How many amps and volts the diode(s) should have? Does it need all those 6 diodes 1N4007? And what the 7th diode does? 
Schematic: http://el34world.com/charts/Schematics/files/fender/Fender_TWIN_6G8.pdf
Here a pic of the rectifier:

[shooter]

I'll see if I can help - some 
Channels, usually individual channels do individual things, *bright, normal, overdrive* each should be capable of driving the PA section.  IF you do multi-channels and want to *jump them together* make sure they have equal gain sections.

speakers, I believe you're right

diodes, yes, 1n4007 is the new standard, you want about twice B+ for piv.  I didn't look at the schematic, so I'm guessing it's half-wave rec.  so the "extra" diodes will just drop a volt or 2.  The "7th" diode ***might*** be for the bias supply.  when I get a minute i'll pull up the schematic.

[HotBluePlates]

1- Channels
  a- One channel alone will do the total output of the amp?

Yes. The preamp channels are simply boosting the guitar's output voltage up to ~1vRMS which is usually enough to enable the phase inverter to drive the output tubes to maximum power.

1- Channels
...
  b -Linking channels. In Fender amps with 2 channels like the Twins, when linked, do they add up to the signal before getting to the power tubes, by linking the channels does it produce more gain?

Yes, but...

When 2 tube stages are in parallel (like linked channels), the total gain is the sum of the individual stage gains (A1 + A2).
When 2 tube stages are in cascade (one after the other), the total gain is the product of the individual stage gains (A1 * A2).

So having a preamp with 3 cascaded gain stages is potentially a lot more gain than linking 2 channels of 2 gain stages each. (60*60*60 = 216,000 vs (60*60)+(60*60) = 7,200)

2- In the amp when there are speaker socket and external speaker socket, lets use as example the 6G8 something I'm familiar with, as far as I understand thethe speaker sockets are in parallel yes? So it is for 4 Ohms load, and there is a 4 Omhs 2x12 cab connected to the speaker socket, what can be connected to the external speaker socket, another 4 ohms cab? But if the sockets are in parallel wouldn't it make those two cabs a 2 Ohms load total?

Yes.

If you want to reflect the same primary impedance as the stock speaker load, then you should have 2x 8Ω cabs connected to the jacks. But the amp will handle a 2Ω load from 2x 4Ω cabs, reflecting a different (lower) primary impedance and probably producing less clean power. However, the extra speakers will probably sound like more acoustic volume in the room, so it's kind of a wash...

3- Rectifier and diodes, In my building of a 6G8 the rectifier I build there are 7 diodes, in the schematic there is no mention about what value they are, so I followed my instincts and what did make sense, and build it with the 1N4007/1000V, 1 amp rating but then reading this topic: http://el34world.com/Forum/index.php?topic=19174.0
...
So it leaves me with the impression that something maybe incorrect in my rectifier. Do i have too much diode value there? How many amps and volts the diode(s) should have? Does it need all those 6 diodes 1N4007?

Yes, you could get away with a single 1N4007 per side (2x instead of 6x), but people generally copy layouts and you're not hurting anything with the extra diodes.

How to determine the diode rating you need? For voltage, take the PT RMS winding voltage and multiply by 2.828. When the PT winding attached to a rectifier is swinging negative, the diode is shut off. The diode will feel the peak of that negative swing (winding RMS volts * 1.414) plus the voltage on the filter cap (which could be as much as winding RMS volts * 1.414). So a 1N4007 with a 1kV PIV rating shouldn't be hooked up singly to a PT winding with more than 1kV/2.828 = ~353v RMS. That implies a peak voltage of 353v RMS * 1.414 = ~500v peak into the 1st filter cap.

The current rating is harder to figure, and is governed by the peak charging current for the filter caps. Rather than guess, most smaller circuit are okay with a 1N4007, while bigger amps might be safer with a 1N5408 (with a 3A rating). You can stack 1N4007's in series to raise the composite voltage rating, but the current rating stays the same. You could add extra diodes in parallel to raise the current rating, but soon you'll spend more than simply buying the diode with a higher current rating.

And what the 7th diode does?

6 of the diodes are for the high-voltage B+ supply. The 7th diode is for the bias supply.

[John]

6 of the diodes are for the high-voltage B+ supply. The 7th diode is for the bias supply.

It looks like he's using bi-polar caps on the bias supply. Does that affect the - voltage at all? (have yet to fool with fixed bias, is why I ask)

[HotBluePlates]

It looks like he's using bi-polar caps on the bias supply. Does that affect the - voltage at all?

No, it just means he paid more for caps than he needed to.

[PRR]

> add up to the signal before getting to the power tubes

Yes. Both channels are just mixed, then sent to the same power output stage.

Linking channels gives a *small* increase of gain, less pluck-force to get the same maximum power.

When the two channels are not identical it may produce a more complex sound, which may be musically useful.

> Do i have too much diode value there?

For some reason, the guy who built this house put three and even FOUR 2x10 floor beams where one beam would do.

Is that "too much"? The house stays-up fine. Wood-cost looks high (but I think he was using-up extra wood he cut on this property). He sure didn't charge me extra for this over-built floor. So nothing wrong with it.

(Actually, the 3 or 4 beams makes it hard work to run new wires or pipes, and crimps space for heat ducts. Likewise your build could free-up an inch of space if you'd done 2-diode instead of 6-diode. And it woulda cost a buck less. However there is NO reason to rip it out and do over.)

> using bi-polar caps on the bias

If so, again (as HBP also says) it is a half-buck "un-needed" cost. A bi-polar can work uni-polar just fine, no change. The bi-polar rating costs a bit more, but not enough to cry over.

[uki]

Wow!!  Many thanks for all the replies!

Lots of good answers  , I just followed the schematic and layout, but as I have noticed, after 50 years of the original production of the Twin-Amp 6G8 lots of things changed in electronics, and I've been learning tons here in the forum.

ok I got it,
1- I can get a little more output by linking the channels with a wire and a more complex tone, that sounds really good to me, experimentantion is my thing.

2- It is then safer to have 2 cabs with 8 Ohms(same number of speakers) each than 2 with 4 Ohms each. Wouldn't the 2 Ohms load blow a 4 Ohms load OT ?    What about cabs with different number of speakers but same Ohms load lets say 8 Ohms, does it have any impact in the sound/output ?

3- Ok no problems with the diodes  , it wasn't in my case just a half buck, shipping to south America is expensive and currency was about 3,5 to 1 when I bought the components, there are no caps down here for sale with those values for amps specially the ones with high voltage rate.

Oh yeah someone mentioned space, that was one of my concerns, I used to have those big heads and big speaker cabs(Giannini Tremendão), too much stuff and weight to carry, so I've reduced the dimensions somewhat in this build.
Here is a good pic of the same model of that old amp, from 60s or 70s. http://1.bp.blogspot.com/-e4-ZCErnPJI/T0r0_uG3k5I/AAAAAAAAA5Y/aOYrcnmR_H0/s1600/P5060044_1690x1267.jpg
Had one of those too: http://img.olx.com.br/images/97/979519089594557.jpg

Remember this is my 1st build.
Here some pics:

[HotBluePlates]

2- It is then safer to have 2 cabs with 8 Ohms(same number of speakers) each than 2 with 4 Ohms each.

No, it's not "safer," but the amp will output the most power when the rated load of 4Ω total is attached (2x 8Ω cabs).

Wouldn't the 2 Ohms load blow a 4 Ohms load OT ? 

No, a 2Ω load won't blow the OT.

Tube amps make the most power when the rated load is attached; they make less power output when some other load is attached. They can tolerate a short-circuit/0Ω load (as in the switch speaker jack on Fender amps), though playing with an open-circuit loads will likely cause voltage spikes which can damage the OT.

What about cabs with different number of speakers but same Ohms load lets say 8 Ohms, does it have any impact in the sound/output ?

Well, speakers have their individual sounds, so the more speakers you mix, the more chance for sound variation. Also, the more speakers you have (while keeping the same load) the louder the amp sounds, up to a limit. You can play a Champ through a 4x12 cabinet and get surprisingly loud.

[uki]

Thanks for the reply!! 

2- It is then safer to have 2 cabs with 8 Ohms(same number of speakers) each than 2 with 4 Ohms each.

No, it's not "safer," but the amp will output the most power when the rated load of 4Ω total is attached (2x 8Ω cabs).

Does the rated load ask more from the amp or stress more the amp/OT, why is not safer?

Tube amps make the most power when the rated load is attached; they make less power output when some other load is attached. They can tolerate a short-circuit/0Ω load (as in the switch speaker jack on Fender amps), though playing with an open-circuit loads will likely cause voltage spikes which can damage the OT.

Do you mean those mono jacks with 3 terminals? I was wondering about that, I did noticed it does short the OT secondaries when no speakers are connected, for how long it can be shorted without speakers connected when the amp is on, there is a limit I guess? I've heard about tube amps are better not to stay on without the speakers connected.


 

[sluckey]

Do you mean those mono jacks with 3 terminals? I was wondering about that, I did noticed it does short the OT secondaries when no speakers are connected, for how long it can be shorted without speakers connected when the amp is on, there is a limit I guess?

Sometimes for days. See this thread...

     http://el34world.com/Forum/index.php?topic=19230.0

[HotBluePlates]

No, it's not "safer," but the amp will output the most power when the rated load of 4Ω total is attached (2x 8Ω cabs).

Does the rated load ask more from the amp or stress more the amp/OT, why is not safer?

Tube amps make the most power when the rated load is attached; they make less power output when some other load is attached. They can tolerate a short-circuit/0Ω load (as in the switch speaker jack on Fender amps), ...

... I did noticed it does short the OT secondaries when no speakers are connected, for how long it can be shorted without speakers connected when the amp is on, there is a limit I guess? ...

I believe you're mixing concepts about speaker loads for transistor amps with concepts about speaker loads for tube amps.

If a tube amp is designed to make 50w with a reflected OT primary impedance which occurs when an 8Ω load is attached to the secondary, then attaching a 4Ω or 16Ω load will result in less power. The tube amp output stage is a balance of tube capabilities, OT reflected primary impedance, and power supply capabilities. So in general, for tube amps the rated load is also the load which delivers the most power output.

A transistor amp usually has no OT, and the load impedance itself limits the current delivered by the output transistors. Say you have an ideal transistor power amp which is rated to deliver 50w into 8Ω. If you attached a 16Ω load, less current flows and power drops to 25w. But when you attach a 4Ω load, more current is pumped into the load by the output transistors, and power increases to 100w; it increases again to 200w when you drop the load to 2Ω.

The reality is that while transistors do exhibit this behavior for higher-/lower-than-rated loads, at some small load impedance you'll burn up the output transistors. So a short-circuit output is deadly to transistors (not so to tubes), while an open-circuit output is perfectly safe for transistors (but could damage a tube amp's OT if you play through the amp without a speaker load).

[PRR]

The lower-power (especially cathode-bias) tube amps pull "full" power from the DC supply all the time. At idle this all goes up in heat. When playing loud, some or most of it goes to the speaker, less to the tubes.

Shorting the speaker, on these amps, does no more harm than idling them. Which they can do for years straight.

The highest-power tube amps (and most transistor amps) pull small DC power at idle, pull more power with signal. The idea is that this added power goes out to the speaker, isn't absorbed by the tube. If you short the high power amps (mostly fix-bias, and especially high power for the number of bottles), you "can" overheat the tubes.

FWIW, I've run some very bold hi-power tube amps, in nasty venues and occasional shorts, and never had a disaster. I do bet if I put a 150W Bogen on the bench with large test signal and shorted output, I could ruin the tubes in hours. (That's easier with 8417 than most of the popular output bottles like 6550 or 6L6GC or EL34.)

While I have fried a lot of transistors in sick or prototype power amps, essentially "all" post-1970 transistor amps have some or plenty of self-protection. We still find a few blown transistors, but most good recent amps can take nearly any abuse and will strain to a safe limit and then shut-down safely.

[uki]

Thanks guys !!

@sluckey, I did read that thread and I can see your point, the amp was fine and just not giving output to the speaker. I did mount the jacks correctly that is one thing not to worry about, yet there are a general check to do before start the final phase, also one thing I did was to check the wires end points for every wire sodered, it was also mentioned on that thread.

@PRR and  @HotBluePlates

Thanks for the explanations, those have been very helpfull, those answers will help me to get things right.

Here a pic of the speaker jacks:

[Willabe]

Picture is not very clear.

I don't see the OT's ground wire.

[uki]

Picture is not very clear.

I don't see the OT's ground wire.

Cheap camera 

you are right the OT wires aren't sodered yet, only the one that goes to the Presence control, green one going up, but you can see OT wires are there if you look closely, there is still few wires to soder, transformers are not sodered yet, before that there are some checking to do, some tests, I'm following this intructions: http://www.paulrubyamps.com/info.html#FirstPowerUp

Maybe this one is better: