Peavey Envoy 110 Trans Tube Beefing up the Output Stage

[aoresteen]

I have a Peavey Envoy 110 TransTube silver stripe (circa 1995) amp.  The lowest speaker impedance load it can handle is 6 ohms.  Is it possible to mod the output stage so that it can safely handle a 4 ohm load?  This would allow me to use an external speaker with the internal speaker (a Weber 8 ohm replacement).

Thanks!

[phatt]

I have no doubt it's possible but Might be a lot less hassle to implement a second powered speaker.
If you try to modify the existing amp you could end up with a pile of electronic junk leaving you with 2 speakers and no amplifier to run them. xP

Maybe if you post a link to the Correct schematic Some of the better qualified folk would be able to quickly give advice.

Phil.

[aoresteen]

Thanks!

The schematic for the Peavey Envoy 110 TransTube silver stripe 1995 is attached.

It is also here:

http://s000.tinyupload.com/?file_id=06874343838168322164

[g1]

  Well, this is the first time I've seen a 6 ohm spec for any type of music gear.  But apparently peavey had a custom 6ohm speaker for this model?
  What speaker is currently in the amp?  Is the weber 8ohm you spoke of in the amp presently or is that what you wanted to add as an external speaker?
  To put it most simply and practically, with a solid state amp if you run it into less than it's minimum rated load, it will overheat and burnout the power amp section.
  Sometimes you will get away with it if you don't play too loud, or add some additional fan cooling.  But other times you will just blow the amp so you really don't want to take that chance.
  The best thing to do would be to wire up the amp so it puts the additional speaker in series with the internal speaker.  Then you will have a little less overall power, but probably not noticeable.  And you will not be in danger of blowing the amp.

 

[aoresteen]

g1,

Peavey sold the amp with their own in-house made 6 ohm speaker.  The reason was so that they could advertise it as a 40 watt amp.  With an 8 ohm speaker it puts out about 35 watts.

I swapped out the Peavey speaker for the Weber 8 ohm. 

Putting the external 8 ohm 4 x 10" cab in series with the internal Weber speaker would put a 16 ohm load on the amp.  How will the power be applied over the 5 speakers total?

Thanks!

[J M Fahey]

Putting the external 8 ohm 4 x 10" cab in series with the internal Weber speaker would put a 16 ohm load on the amp.  How will the power be applied over the 5 speakers total?

It will be distributed very poorly and to boot you will lose 50% power, so not recommended at all.

I suggest you add a plastic (insulated from chassis) switching external speaker jack, so when nothing is plugged in, you use the internal Weber; then when you plug the 4x10" the internal gets disconnected and the external gets full power.

In any case you enjoy full 35W RMS.

Much better than having 17.5W total unevenly split over 5 speakers.

[g1]

  Agree with JM.   I was unaware you were wanting to use a multi-speaker cab.
To run all 5, the amp speaker would get half the power, and the 4x10 cab would get half the power.  Not much point in that.

[Roly]

{agree, agree}

so that they could advertise it as a 40 watt amp.  With an 8 ohm speaker it puts out about 35 watts.

And that is about where it begins and ends - marketing.  The audible difference between 35 and 40 watts into the same speaker is nothing, about half a dB, and much larger gains could be had by simply using a speaker with a higher power conversion efficiency (dB/W), then you might get an audible difference between 35 watts and 35 watts.   

[aoresteen]

Putting the external 8 ohm 4 x 10" cab in series with the internal Weber speaker would put a 16 ohm load on the amp.  How will the power be applied over the 5 speakers total?

......

I suggest you add a plastic (insulated from chassis) switching external speaker jack, so when nothing is plugged in, you use the internal Weber; then when you plug the 4x10" the internal gets disconnected and the external gets full power. ....

Thanks!.  This model Envoy (silver stripe) already has such an external speaker jack.  I guess that's what I'll have to do.

[aoresteen]

I've done a bit more research on the Peavey Envoy 110.  The power amp stage uses Peavey power transistors 70473100 (PNP) and the 70483100 (NPN) in a push pull configuration.  These parts are really Motorola MJ15015/MJ15016 amps:



My thoughts are to replace them with more robust power transistors such as the Motorola MJ15024/MJ15025 power transistors.

Would the MJ15024/MJ15025 transistors be able to handle a 4 ohm load that would produce about 60 watts?  And would I have to upgrade other compoments in the power amp stage?

Thanks!

[g1]

  What you are really asking the amp to do is deliver more current.  To accomplish this you would probably have to upgrade the power transformer, maybe other power supply components, and heatsink (or add internal fan).
  It's not really practical, would be much more economical to upgrade to a higher power amp.

[Roly]

Agreed.

From your pix that amp essentially doesn't have a heatsink, hence the thermal cutout on the "transistor holder".  And that is just the start.  You will need a bigger power transformer, possibly rectifier, certainly filter caps, a real heatsink, and perhaps a liquid nitrogen dispenser.

I also wouldn't use that as a starting point where all the components have obscure in-house numbers - it would be hard enough with a fully documented amp.

At the end of the day it would be a lot easier to trade it in on a bigger amp.

[aoresteen]

Thanks guys!  I have done some more research into larger amps and I am now confused.  I found two other amps of the same vintage that have higher power.

1. Peavey Studio Pro 110 silver stripe - 65watts

2. Peavey Bandit 112 silver stripe - 100 watts @ 4 ohms, 80 watts @ 8 ohms

Both amps use the exact same power transistors that the Envoy 110 does - 70473100 and 70483100 (MJ15016 and MJ15015).  This tells me that the Envoy 110 can produce 100 watts if it has the proper power supply components & a speaker that can handle the power.  No need to swap them out (but I will need a much larger heat sink ect).

Both amps are rated at 8 ohms only - 4 ohms are not recommended for either of them. (Edit:  Error!  The Bandit will take a 4 ohm load - external speaker can not be less than 8 ohms,  my bad!)

The Envoy 110 power amp is driven by 15.5v rms.  (15.5^2)/6 yields 40 watts @ 6 ohms as Peavey states.  40w/15.5v yields 2.58 amps of current.  If I do the calculations with a 4 ohm load I get 60 watts and 3.875 amps of current.  I do not know what the Envoy power supply is rated at but I will assume it's less than 3.875 amps and greater than 2.58 amps.

Doing the same calculations for the Studio Pro 110 which is driven by 22.8v RMS I get 65 watts as Peavey states and 2.85 amps being drawn.  At 4 ohms the load would be 5.7 amps for 130 watts.

I hope that so far that my math is correct.

Here's where I get confused.  The Bandit 110 is rated at 80 watts @ 20v RMS @ 8 ohms.  How does Peavey get the extra power?  At 20v RMS it should produce 50 watts with an 8 ohm load but it produces 80.  I've attached the Bandit 110 TT schematic which shows one pair of power transistors.  I must be missing something.

The Envoy's power supply is rated at 75 watts (.625 amps) at 120 VAC, the Studio Pro is 200 watts (1.67 amps) at 120VAC, and the Bandit is 300 watts (2.5 amps) at 120VAC.  Is there anyway to calculate the rated amps at the drive voltages like 15.5v RMS?

Also, if I dropped the Envoy's v RMS from 15.5v RMS to 10.5v RMS, I calculate that at 4 ohms  I would get 27.6 watts and draw 2.6 amps which is in the Envoy's power supply range.  Would this work?

Thanks!

[J M Fahey]

Here's where I get confused.  The Bandit 110 is rated at 80 watts @ 20v RMS @ 8 ohms.  How does Peavey get the extra power?  At 20v RMS it should produce 50 watts with an 8 ohm load but it produces 80.

Your Math is fine, just don't blindly trust what the Marketing dept. guys say.

That model is, as sold, a honest 50W/8r amp which will put out 80W RMS ... if you add an external 8 ohms speaker.

Most 150W Bass combos are actually 100W RMS as sold, etc.

Just read the fine print, such as here:



As of your other calculations, the "software" (calculations themselves) is perfect, but the "hardware" (transformers/caps/diodes/transistors/heatsinking/space) does not support what you want.

To be more precise, you ***might*** be able to add a real finned heatsink mounting transistors there, add a small 24V fan with a series resistor to cool the PT and diodes (doubt you can mount a significantly larger one there) , replace the speaker with a 4 ohms one, to get what? .... 50W RMS?

Practically impossible to distinguish from 35W RMS, unless you switch back forth *quickly* and then difference will be quite subtle 

I doubt there is a Club (or loud drummer) which you can not meet with 35W but 50W will be ample.

IF you actually need , say, twice as much perceived power, build an extension POWERED cabinet, with a nice 12"guitar speaker and its own power supply and power amp, an LM3886 chipamp is about perfect (50/60W RMS on its own) and WILL make a difference.

FWIW such a product is commercially available (and very succesful), enter the Tech 21 Power Engine 60
http://www.musiciansfriend.com/amplifiers-effects/tech-21-power-engine-60?_requestid=74531
which is exactly that

https://www.youtube.com/watch?v=lOpFmaaIOps

and as they say:

"the Tech 21 Power Engine 60 is an open-backed 1x12, 60W powered extension cab designed to be used with the Trademark 60. It is not a standalone piece."

Of course, you can build yours for much less than U$419

EDIT:  FWIW In sold TONS of such powered cabinets



mainly to guys who believed the fair tale that "tube Watts are bigger than SS Watts" and bought 15/20W 2xEL84 amps , then were drowned by drummers, also others who bought, say, a POD or some multi multi effects pedalboard, set it to preset#47: "Jimi Hendrix with 6 Marshall full stacks playing full tilt at Woodstock" or preset #92: "Metallica playing full tilt with 6 Mesa Boogie Dual Rectifier and 12  4x12" cabinets" and "somehow" didn't get that exact sound when plugging into an Envoy 110 or similar  :loco

Car riders and bikers have a saying: "there's no substitute for cubic inches"  .... which here can be translated as:
"there's no substitute for (speaker) square inches and (amp) watts" so the more you add, the better.

[g1]

 A few comments.  You mention "power supply rating", but that is the rating of what the amp uses from the wall outlet.  Just ignore that number, there is no way to say how efficient the amp is, only we know it will be less than 100%.  You can't really get any useful info out of the 120V rating as far as power output.
Now, as to the RMS values given for various outputs, it will change depending on the load impedance.  The RMS into the speaker is a function of the DC supplies to the output transistors.  This will change depending on how much current is being delivered to the load.  You won't get the same RMS into a 4 ohm load as you will into an 8 ohm load.  This is because the power supply will "sag" when asked to deliver more current.
  Look at the Bandit for example.  80W@8ohms.  This works out to 25.3VRMS as stated.  But they rate 100W@4ohms.  Working backwards from that, you see this is only 20VRMS.  This is because the power supply sags when asked to deliver more current.
  The RMS value is a function of the DC supplies.  Converting 20VRMS we get roughly 56V peak to peak.  That would be the voltage available to the output transistors from the DC supplies at the time of measurement.  So at full power into 4ohms there would be 56VDC between the + and - supplies, or +/-28VDC.
That is what you would measure at C72 and C87.
When the Bandit is running into the lighter 8ohm load, the supply does not sag as much.  The 25.3VRMS works out to approx. 70.8V peak to peak.  So there would be +/-35.4V at the supply rails at this time.
  I hope this shows how the RMS voltage does not stay constant as the load changes.
  So the Bandit will run higher supply voltages to the 2 transistors, that is why it can deliver more power than the other amp that has the same transistors.  To get the extra supply voltages you need a bigger power transformer.
  To run the transistors at higher current and power, you need better heatsinking to keep them from burning up.