Single Ended Amp 8 Ohm To 4 Ohm Conversion

[jsaults]

Hey people! I have a question regarding a single ended bass guitar amp that uses two nte 130 or 62792 output transistors.

The schematic is available on peaveys web site under the manual for tnt 100. These are driven via a dual secondary transformer.

What I am trying to do is drive a 4 ohm load with this amp and the spec.s show that it is 45 watts into an 8 ohm load and 25 watts into a 4 ohm load. I was wondering if there is a similar transistor that would better serve the 4 ohm load and get the power back up to 45 watts without much more modification than swapping the transisitors. I know this is a shot in the dark but I seem to remember someone else doing something similar once. Thanks for ANY input!

Justin

[Elvee]

Hi

It seems the power limitation in 4 ohm comes from the limited output current the transistors can provide, given their drive level and beta factor. A possibility would be the replacement of the originals transistors with higher gain models. If you choose a larger transistor, it will have a higher gain at the same current level. Examples of such replacements: 2N3772 or BD249 (not identical, but mechanically compatible).
But before attemptig anything, I'd check the real behaviour of this amplifier on a 4 ohm load: the guys at Peavey are serious designers, and like any serious designer, they build in comfortable safety margins in their products to account for part-to-part variation. This means that unless you've stumbled on the worst lemon of the series, your amplifier will be able to put out significantly more power than the minimum specified. Only if this is still too low can you envisage modifications.
LV

[richie00boy]

Seems to me that you got the 25 and 45 the wrong way round. 4 ohms being a tougher load will drain more power. It's ohms law.

[jsaults]

Yeah I thought that myself but I am quoting that from the manual. I know I have seen other amps like that. Here is the link to the manual. http://www.peavey.com/media/pdf/manuals/tnt100.pdf

Thanks for the info on the other transistors! I'm not sure what I'm going to do yet, maybe I'll try the different transistors and keep an eye on current flow and heat and see how much it changes.

[richie00boy]

It's got to be a misprint. You don't need to do anything to your amp, just hook up your 4 ohm speaker and start playing

[Elvee]

Originally posted by richie00boy
It's got to be a misprint. You don't need to do anything to your amp, just hook up your 4 ohm speaker and start playing

It is not a misprint: the output current is limited by design, although most of the exemplaries will be able to put out more power; and the supply transformer is dimensionned for 45W, not 80 or 90.
LV

[jsaults]

I should be ok with the transistors you mentioned though as far as current load on the transformer goes or do I need to upgrade the power supply? Thanks for all your input!

[Elvee]

If you're too successful with your mods and you get at over 45W, the transformer might become an issue, especially if you play at extremely loud volume with max fuzz & dist. for extended periods of time. Btw, you should also reinforce the filter caps; 1000µF is very much on the light side.
LV

[jsaults]

Thanks I'll watch the levels. I do have some larger caps on hand, I'll jump up a few thousand, they're ancient anyways.

[AndrewT]

Hi,
Peavey are likely to have scaled all the expensive components to run out of capacity at near the same output power.

Dropping the load impedance could cause the bottleneck to be in the output stage, the drivers to the output stage, the heatsink on the output stage, the smoothing capacitors and/or the mains transformer.

If you increase the capacitors to provide more smoothing then the output devices will have a higher average voltage across them while still outputting upto the pre-set current limit. This will cause more heating of the output stage and bring the devices closer to the SOAR limit due to higher Vce, and that is the SOAR that has just got lower due to the extra heat. A double whammy.

Lower impedance than specified is a hard load that breaks amplifiers. Go easy (while playing!!).