That said, one of the
satisfying things about all-valve amps is their comparitive
simplicity. Only 5 stages of amplification and the tiny guitar
signal is beefed up to potential pain level; and each of those
stages is fairly easy to change. In modern solid-state amps, so
many chips are used, each one containing hundreds of
transistors, the simplicity is lost, the sound must suffer, and
experimentation is very difficult.
Read up a little on who makes
valves these days. Despite there being many brand names out there,
it usually boils down to three factories; one in China, one in
Russia, and one in Slovakia. It's embarrassing to get excited
about the possible differences between 2 brands and then discover
the only difference is in the print job. You can pay more for more
testing, and that may be a good thing, but don't forget, this only
eliminates the ones you didn't buy - it doesn't improve the tone
of the ones on sale.
I should probably add that,
the more I discuss and read up on this, the more I get the
impression that the speaker's more important. If your present
valves are working OK, and not old and tired, then changing the
speaker will have a far bigger impact on your tone. (Unless you
know different; email me...)
Myths about Valves
(1) 7025s are
No, they're not. They were
originally devised as a low-noise 12AX7 (with coiled heaters to
reduce hum), but in reality any reasonable-quality ECC83 or 12AX7
will work OK in the three places where 7025 is specified. My PRII
is unbelievably low-noise with every 12AX7 I've tried.
(2) Don't touch the glass
You shouldn't touch them when
they're hot because you'll be hurt! Duh! But these aren't halogen
light bulbs, and the sweat from your fingers won't harm the valve.
But don't shock them mechanically while they're hot. Frequent
handling can rub the ink off, which might reduce the secondhand
value of a famous-name valve. And try to hold power valves by the
plastic base when fitting/removing from the amp, so reducing the
risk of detaching the base from the glass.
(3) If you use low-gain preamp valves, the power valves
have to work harder, which is a good thing
No. If you use low-gain preamp
valves, you may or may not get a different tone, which you may or
may not like. Then, to get the same audio volume as before, you'll
have to turn the master volume knob up to a higher number on the
dial. But if it's the same audio volume as before, then the power
valves are doing exactly the same amount of work as before. If you
want the power stage to work harder without an increase in audio
volume, you'll have to use a less sensitive speaker or throw away
some of the final output with a power attenuator.
----------------------What the valves (tubes) do
V1 on Schematic - Input
valve (7025 = ECC83 = 12AX7)
This one is close to the serial number plate. It's used as two separate stages of amplification in one glass bottle. One stage provides the first amplification after your weak little guitar signal arrives, almost exhausted, at the input jack. NB the amp's volume control comes after the first 'half' of this valve, not before, so it's not just a duplicate of your guitar's volume knob. The comes the bass, mid, and treble controls, and the volume knob; the other 'half' of this valve then provides another stage of amplification. How the tone stack works
Note that , when there's no
jack plug in the front panel socket, the inputs of both halves of
V1 are grounded by a switch in the jack socket. This cuts down on
Obviously you want all
the valves working well, but the signal passes through this valve
twice, while it's still at a low level; so if this one's a
bit noisy and tired, the others won't be able to help no matter
how great they are. If you're minus a valve cover, don't let this
valve go without, as in theory it's the most sensitive to
Oct 2010 - I'm now using a
12AY7 in this position. On an arbitrary scale, 12AX7s are rated
'100' for gain - 12AY7 are rated '40'. This cuts down the gain and
ultimately the maximum volume you can get out of the amp, but (on
my amp at least) it makes the clean sound much cleaner and takes
away the feeling that there's still something a little
unintentionally overdriven in there. This is not the same as
simply turning down the front-end volume control, though I can't
explain why! Using a lower gain valve here also cleans up the
reverb a little. Anyway, 12AY7s are in current production by
Electro-Harmonix and are widely available. I got mine from Voc Rock
in Nottingham, UK - they keep some in stock even though they're
not always shown on their website. It's not difficult to find NOS
V2 - Reverb Driver and
'lead' distortion (12AT7)
This valve is also a dual
triode but the 2 little amplifiers are wired in parallel to create
one preamp with enough current-capability to function properly. It
has 2 jobs.
(1) It amplifies the signal considerably in order to wiggle the springs in the reverb unit. And it's doing it all the time, even when you haven't selected reverb - the 'reverb' knob comes after V3a (below). BUT
(2) irrespective of reverb setting, when you select the 'lead' effect, most of the (massive) signal from V2 is mixed into the sound via V3b, resulting in V3b being distorted (= "preamp distortion) and offering you a kind of valve overdrive sound.
Therefore the reverb depth
drops to near-zero when 'lead' is also selected. This is a
universal problem on the PRII and I know of no cure. Even if the
reverb level could be kept up (maybe with some component value
changes?), it would be a reverb of the clean sound only. Ideally
the reverb would come after the distortion stage, so you could
simulate "an overdriven amp in a big room", but without a redesign
this amp will not do that.
When in 'clean' mode
(non-lead), the PRII has a reputation for useful reverb only from
knob position 1-3; turn it up higher and all you get is more mush.
If your PRII has the reverb tank mounted vertically,
screwed to the side wall of the cabinet, put it in the base of
the cab instead. The tank was designed to be horizontal and
the reverb quality will improve bythis
simple operation. But I found by
accident that different 12AT7s can affect this, sometimes
improving it. Maybe a lower-gain valve gives more useful
adjustment; I know of one owner who has replaced this valve with a
12AU7 for this reason. But that will also affect the 'lead' sound.
The amp will work without this valve - but minus reverb and lead
(overdrive). I used to say on this web page that you needn't
bother putting an expensive valve here. But I was wrong. In Feb
06; I put an old Mazda 12AT7 in here (with an old Mazda 12AX7 in
V3) and the lead sound is much better. Then I put a used Brimar
12AT7 in V2 and the "lead" sound is now very usable. So don't let
anyone tell you the "lead" effect is useless; you just have to be
lucky with your valves. I buy used valves from eBay, sometimes
without even a guarantee that they work. They cost one or two
pounds each for Mazdas, Brimars and Mullards, and they nearly
V3 - Reverb Pickup and
final preamp (7025 = ECC83 = 12AX7)
(Nearest 'pedal red' socket.) Like V1, this works in two 'halves'. The first 'half' picks up the signal from the reverb unit and passes it on via the reverb knob. The other 'half' provides another stage of gain for the main signal path through the amp. This second 'half' is the bit which is purposely pushed into distortion when you select 'lead', so swopping this valve will change the way that effect behaves. (Whether you like this kind of distortion (ie in the pre-amp), or prefer power amp distortion, is up to you; they are different.) I have an old Mazda 12AX7 in here and the lead sound is much better.
After V3 comes the master
volume knob, the lead level knob, and the presence knob. NB the
line/recording output comes after the output valves, not from here
or anywhere in the preamp chain.
How the presence control works
There's a link from the
secondary of the output transformer. it comes off the leg labelled
'grn' on the schematic and runs back over the top of the
schematic, via a 100K resistor, to the inputs of v4. Below the
inputs to v4 there's potentiometer (the presence control)
with a 0.1uF capacitor going down to ground. The link back from
the output transformer is the Negative Feedback Loop or NFB. It
feeds a small amount of final amp signal (loudspeaker signal) back
into the earlier circuitry, but the feedback is out of phase -
it's positive when the signal coming through the amp from v3 is
negative, and vice versa. The result is some cancellation - a
reduction in gain - and some reduction in distortion, because any
errors introduced in v4, 5 and 6 are cancelled out a little.
(Some folks reduce the
amount of NFB (eg, increase the 100K resistor) or get rid of it
completely (cut the link at some point). The effect is to make
the sound 'looser' with more distortion, more noise, more
character and the possibility of the thing going unstable and
starting to oscillate (hooting with no guitar connected). I have
done this and it's fine simply with disconnecting that resistor.
The presence control is a tone
control in the loop.
Because it's a negative
feedback loop, it has the opposite
effect to what it should if it were located in a normal
part of the schematic. Turn the presence up = reduce the
potentiometer resistance = send more highs in the loop to ground =
don't cancel out the nornal-signal highs heading for v4 = more
highs in the final output, adding a bit of zing that might
otherwise get lost.
V4 - Phase Inverter (7025 =
ECC83 = 12AX7)
This one splits the signal so that both the signal and its mirror image are sent to the pair of power valves, so they can do their push/pull thang. I read somewhere that gain isn't a critical factor here. However some folks say you want a 'matched' valve/tube as your phase inverter if you want the maximum clean volume. This means the valve is matched with itself - there are 2 separate gain stages in a 12AX7; they are supposed to be identical but of course they usually aren't. Some suppliers will test for good matching of the 2 gain stages and sell the valve as 'matched' (at a higher price). This doesn't seem important to me, especially since the other components are 10% tolerance and not "matched", but I'd be pleased to hear from you if you've investigated it.
V1-V4 do not need to be
matched to each other in any way, and when you change them, no
internal adjustments are needed. V1,3 and 4 (NOT V2, to be true to
the original design*) may be interchanged with each other - the
amp will work OK. If these valves come from different
manufacturers/batches, the response, tone and noise levels will
change as you swop'em around. Turn off the amp and allow to cool
before changing them, though.
*Note May 05 - I should have
added this bit some time ago, once I realised you can kind-of
interchange 12AX7s and 12AT7s. There won't be any damage but the
amp will behave very differently. You might like it; you might
not. Basically the difference is the 12AX7 (used throughout the
main signal path) has a higher gain but lower current capability.
The 12AT7 was deliberately specified by the amp designer for the
reverb driver because more current and less gain is needed at that
point. So theoretically a 12AT7 in V1,3 or 4 will reduce the amp's
overall volume capability in return for different tone. As already
noted, a 12AX7 in V3 has an effect which some folks like, though I
would expect it to die early because it's trying to supply too
V5 and V6 - Output or Power
Bigger and fatter than the other valves, and lining up with the back panel where it says 'Princeton Reverb II'. They don't have covers, as they're barely susceptible to interference, and need all the ventilaton they can get. They work as a team to push and pull current through the output transformer, which in turn wiggles the loudspeaker. (At the risk of stating the obvious... the amp needs both of these valves to work; if one fails, you lose more than half of the power, with a truly horrible tone.) These 2 valves are best replaced as a matched pair; even then the amp needs checking, and maybe adjusting, as described in Rebiasing the output valves/tubes. Rebiasing isn't simply about nice tone, it's also about not destroying your new power valves.
These are specified as 6V6GTA.
In practice you can use anything beginning 6V6 except those with
metal casings. Metal-casing
6V6s have three problems; they're often microphonic (relaying
mechanical vibrations as sound out of the speaker), they were
designed with lower plate voltages in mind (so are more likely
to fail) and they were sometimes constructed so that a high
voltage might appear on the casing (so they're unsafe). Use
only glass tubes, which what the GT stands for.) CV511 is an
alternative name for 6V6GTY, which is what I'm currently using.
GT, GTA, GTB, GTY... a little work on the web will show you what
the difference is. GT's are rated about 15% less power than the
others. In practice they're all fine so long as (a) you like the
sound they make (b) the bias isn't adjusted so hot that they self
If the output valves aren't
matched to each other, the amp will still work, but in
extreme mismatch cases (1) it's not as efficient (2) you might not
be able to get a clean sound, and (3) one valve will age
faster than the other. But a small mismatch doesn't matter, in my
opinion. I have bought unmatched pairs, tested them on a home-made
bias probe, found them to be further apart than a matched pair
should be, and yet run them with no audible problems at all.
In techno-speak, this
amp has a pair of 6V6 valves (tubes) in a class AB output stage
with fixed bias. The word 'fixed' here doesn't mean "you can't
change anything" - it means you have to take steps to adjust
them. The other type of amp design is called cathode-bias;
that's self-regulating (bias doesn't need adjustment), but it's
not used in the PRII. The combination of class AB and fixed bias
gives the most power - in other words, you're probably not going
to find a louder 2 x 6V6 amp.
I'm told V5 and V6 can be
replaced with 6L6's (which are normally fitted to larger amps).
You get more power but at a cost, a 4 ohm speaker is needed
instead of 8 ohm, and the amp MUST be rebiased, no exceptions; see
page for details.
Try to hold power valves by the plastic base (not the glass) when fitting/removing from the amp - it reduces the risk of detaching the base from the glass.
I use a small clip-on fan
(made from old computer parts) to run extra cooling air over the
power valves. It can't make any difference to the temperature
inside the valves - if it did, they wouldn't work properly! But it
does reduce the amount of heat flowing into the rest of the amp.
Most of the works live inside an unventilated metal box which is
accidentally heated by the power valves, and that can't be good
for them, so anything which blows heat away will be A Good Thing.
There's the output transformer. The loudspeaker and the line out/recording socket both come off the output side of the output transformer.
... and that's it. On a really
classic all-valve amp you'd have one more valve, i.e. the
rectifier, but in this amp rectification is achieved by 4 silicon
diodes. There are advantages and disadvantages with each.
"There's a lot going on in that
area of the circuit. They were obviously really trying to
enhance the tone.
Like the common BF and SF Fender tone stack, the Treble control acts as a balance control for the highs and the lows. On the schemo I have, there is an error: the 100k should be shown connected to the plate of the tube and the 250pf. The signal is split there; the highs like to go through the 250pf to the 'upper' leg of the treb pot, the lows are resisted there due to that small 250p cap. The lows however more easily go thru the 100k slope resistor than the highs and divide up between .1 bass cap and .047 mid cap, and get drained off to ground according to the position of the bass and mid pot. The highs don't like to go through resistors as easily as lows, so they are attenuated slightly. If the mid and bass pots are set high, then more of the signal gets put on the 'lower' leg of the treb pot. Thus explaining the standard Fender tone stack and why the treb is a balance pot between highs and lows. So now, all the signal is coming out of the wiper of the treb pot, and sent to the vol pot via a network of caps and res, where more is happening that you don't normally see on a Fender tone stack.
The volume pot is center-tapped. When the volume is at half way, max highs through the 120p and 100p are sent direct to it, less if it's on either side of half way. signal is sent thru the 820k slightly attenuated in the highs again due to the resistor, and goes to the upper leg of the vol pot. The 820k resistor in series with vol pot also loses some volume at the top of the vol pot, and remember some of the highs are already held back in addition cuz of that 820k res.
Now you have another leg going thru the 500p and the 1m, the one meg is shorted out when the push switch on the treb is closed effectively putting the 500p direct to the top of the vol pot, so when the vol is dimed there is a direct line from the treb pot thru the 500p (the 500p holds back the lows from getting thru) to the top of the vol pot.
So there should be an apparent tone shift through the range of the vol pot.
There is a circuit to bypass the 250p treb cap which adds more mids.
(This message was last edited by grkeith at 06:18 PM, Mar 9th, 2007)"
grkeith, who wrote the
above, has his own amp
repair business in Grand Rapids, Michigan, USA.
You can see this circuit working (but not the pull-boosts) if you play with the component values in the 'Fender' section of this excellent free downloadable software.
I hadn't thought of is that the entire signal of the amp
passes through the treble control. So don't bang THAT knob
on a door frame, eh?
<>The 12DW7 lowered the gain on the drive channel but made the reverb more intense because the low side of this tube is pins 1,2 & 3 and the high side is on the reverb driver. I experimented last night substituting different tubes in V2 & V3 and I ultimately decided that a 12AU7 in V2 and a 12AX7 (ECC82 & ECC83) worked the best. I tried a 5751 in V2 & V3 which sounded good, and I have a few more combinations of 5751, 12AU7, 12AX7, and 12AT7 that I have yet to try, I'll keep you posted....
I tried different combos of lower
gain tubes in V2 and V3 and found a winner. I also lowered
the gain on V1.
V1 Sovtek 5751
V2 Tung Sol 12AY7
I decided to drop the value of V1 because I don't realy play that loud and I'm mic'ed mostly always. It's plenty loud, but I get the clear, crystal glassy Fender thing on the clean channel and if I select the lead by pulling the switch on the amp and also pulling the mid boost I get a nice break-up with lots of mids that will slice through any mix (Think Allman Bros.) with humbuckers, with a stratocaster (I have a Tokai that I absolutely love) the lead channel gives me an edgy, but pleasant blues tone I can surely live with. Now I'm ready to gig this amp. Like I said before I've been in the realm of EL84 class A Vox type amps and although they do what they do they don't come close to the clean tones that you can get from a Fender. The amp sounds phenominal with pedals. (Teese RMC2-Micro Vibe-69 Fuzz-Bad Monkey-Barber Direct Drive-Klon-Supa Trem- Boss DD3 Delay---Amp). It speaks with authority, I'm lovin this amp.Thanks, George.
contains a great roundup of 12AX7 and 6V6GTA reviews.
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All trademarks are acknowledged – Fender, their amp model names, Rivera, Krispy Kreme.
Valve (tube) amps develop LETHAL VOLTAGES while running, and store them in charged components EVEN WHILE SWITCHED OFF AND DISCONNECTED FROM MAINS SUPPLY. These voltages are MUCH HIGHER than mains, and higher than anything you’ll find inside a transistorized amp. If this scares you, good. Inside a chassis, don’t use your fingers to touch anything which isn’t insulated or earthed (grounded). Don’t stick more than one hand in at a time, and keep the other hand well away. Use fine-nose pliers to manipulate components. Never, ever work inside a live amp while holding a connected guitar. If you’re not sure what you’re doing, get local help.
The details in this whole site are believed accurate but you act on them at your own risk. I have to disclaim any responsibility for injury, damage, loss of value or loss of gig due to inoperative equipment. Many of the web pages linked from this site say roughly the same thing, and their content is of course not my responsibility.