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Fixing An Ampeg B25 Problem

By Sid of Stone Marmot

Oct. 24, 2008

The Ampeg B25 is a great vacuum tube bass amplifier that was manufactured from the late 1960's to the mid 1970's. It also sounds quite good with guitar. It was conservatively rated at 55 Wrms, though many measure around 70 Wrms. It does have one subtle though potentially very serious design problem: The low frequency response is TOO low.

There are four places in signal chain, other than the output transformer, where the low frequency bandwidth is limited. (See for the schematic of the amp.) These are:

Stage Channel 1 Channel 2 Common Frequency
preamp C2 C9 - 1.6 Hz
tone control C6 C13 - 7.2 Hz
phase inverter - - C17 1.5 Hz
power amp - - C19, C20 4.8 Hz
whole amp - - - 8.9 Hz

When you combine these cutoff frequencies by the root-sum-square method (the correct way to combine these frequencies), you get an overall cutoff frequency for the amp of about 9 Hz. This is less than half the lowest frequency (about 20 Hz) a healthy human can hear. It is much lower than the lowest note (about 41 Hz) on a standard four string bass guitar.

This does not include the cutoff frequency of the output transformer. I don't know the low frequency limit of the output transformer. But due to a test I'll describe shortly, it is definitely extremely low and has little influence on these results.

Why is this a problem? One reason is that it is that we are wasting energy and headroom amplifying these subsonic frequencies that no one can hear anyway. This causes the amp to saturate sooner and more often, reducing its apparent volume capability.

But a much more serious reason is that it damages speakers. The lower a frequency, the more a speaker has to move to reproduce this frequency at a given volume level. This speaker cone movement is usually referred to as cone excursion. If there is too much cone excursion, the speaker cone will start start to physically distort. The voice coil will no longer move straight in and out but will start to tilt. The magnetic gap that the voice coil moves in is very narrow. When the coil starts tilting, it can rub against the walls of the magnetic gap. This can short out and burn up the voice coil, cause it to catch and rip loose (separate) from the cone, or a number of other bad things. In other words, excessive cone movement can damage speakers. This excessive cone excursion is probably the number one reason speakers fail.

It is easy to demonstrate this potential problem with the Ampeg B25. Plug a bass guitar into a normal input, turn the amp on with a reasonable volume level, and simply push the low E string up and down fast. Don't pick it or pluck it or play it in any way, just push the string. Watch your speaker as you do this. You will see the speaker move appreciably in and out. This is not good. Simply pushing on the strings or especially slap playing could damage your speakers.

(This also demonstrates that the B25 output transformer has a very broad low frequency response. Most tube transformers can't pass such low frequencies. This shows that Ampeg didn't cut corners in the design and build of their amps, which may also explain why there is still such a big demand for these old Ampeg amplifiers.)

This problem is also present in the Ampeg B-22-X and B-42-X. These amps use the same basic circuit as the B25 and even have the same reference designations for the components in question. The difference between the B25 and these other amps is that the B-22-X and B-42-X have other circuits added, such as reverb, to make them better suited as guitar amps. I haven't seen this problem in any other Ampeg amps.

How do we fix this? One way would be to plug your instrument into a high pass filter (a filter that only passes frequencies greater than a certain frequency, say 30 Hz in this case) before plugging it into the B25. This is what I did for a while after I first figured out why my 55 Wrms amp kept damaging speakers, even 200 W speakers.

A better way is to change one of the caps inside the amp. I would recommend changing C17. This way you only need to change one cap to affect the response on both channels. If you change C17 from 0.1 microfarads to 0.0047 microfarads, you will limit the low frequency response of this stage to about 32 Hz and the whole amp to about 33 Hz. This will still comfortably pass the lowest note on a standard four string bass. This same change will also work on the Ampeg B-22-X and B-42-X.

Doing this change inside the amp may affect the vintage resale value of the amp. But my feelings are that this stuff is meant to be used, not as museum pieces. Besides, you can always change it back before you sell it.

Notice I haven't shown any pictures of where these parts are located. Anyone with lots of electronics experience should be able to quickly find the parts based on the schematic. If you don't have that kind of experience, you probably shouldn't do these modifications yourself as some of the voltages in these amplifiers, which can be stored for a long time on some of the capacitors even after the amp is off and unplugged, are around 500 V and can be VERY LETHAL! Do yourself a favor and take your amp and these instructions to someone with experience to do the mods for you.

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