Tweed Deluxe Coupling Cap Changes

[Chodrick1]

I've got a 5E3 kit from Mojotone that I am looking to tinker with. I have read a number of times, that in order to reduce the boomy low frequency that the 5E3 is known for due to the use of .1uF coupling caps, you can reduce this number by half or more using .047uF or .022uF coupling caps, this restricts the low end from moving through the circuit.

You may do this by replacing the caps in the preamp, or you may do it by replacing the caps in the phase inverter. So my question is,
"What would be the advantages or disadvantages of replacing the caps only in the preamp versus only replacing the caps in the phase inverter?"

It would seem to me that the amp would benefit either way.

If you leave the .1uF caps in the preamp and modify the phase inverter, this will allow you to shape tone more dynamically seeing as all of the frequencies are moving into the tone stack, and you may adjust all of them from there. It should give you a much larger hold on your tone seeing as you are not limiting it. (Wider tone spread)

On the flip-side, replacing the caps only in the preamp would make it to where you are limited to only a specific range of frequencies to move through to the tone stack, severally (or maybe not) limiting your ability to shape your sound. However, it limits you to only the range that you have decided, which would help you remove any other frequencies that you don't want. (Fine tuning of the usable spectrum)

I'm just typing out loud, but from what I have seen, it is much more common to replace the coupling caps in the preamp. However on The Edge Signature Tweed Deluxe, they opted to leave the stock caps in the preamp, and alter the phase inverter instead.

I've gone on long enough, but I was just interested in how changing the preamp will effect the amplifier, opposed to how changing the phase inverter will effect it. Also let me know what you prefer, I love to hear suggestions.

Thanks!

[Jazz P Bass]

Personally, I would change the first tube cathode bypass cap.
25uf can be lowered to 10 or even 4.7uf.

[Chodrick1]

I have been told that as well. I replaced the 25uF with an 8uF cap, it was the lowest value I could find on Mojotone. It seems to really brighten the amp and I do like the changes that it makes. Ill see if I can get my hands on some even lower values from Mouser or something. Luckily its a rating of 25VDC so they arent hard to come by.

[TimmyP1955]

My boss has a BFPR on which the coupling caps were changed from .1 to .01. It sounds great.

[Chodrick1]

I havent heard of using .01uF caps, that seems petty low, but it would probably sound great for clean and overdrive tones. I like my amps to be brighter, I love the crisp, bright and full tone of my Deluxe reverb, and I freaking love the tone of Princetons, Tweed, Blackface, Brownface, they all sound wonderful.

My Deluxe has a hint of that, Im using .047uF on the Bright channel and .022uF on the Normal channel. I also had to put a bypass cap (bright cap) across the volume control on the Normal channel. It would sound so muddy and nasty when crabked, and not in a good way. I think I used a .01uF cap actoss the terminals. It sounds nice, but somehow it feels really loose and bright. Almost more than I would desire for my dream tone.

[Chuck H]

I recently did an experiment with an amp design that I previously used a partial cathode bypass for on the first gain stage. This amp is intended to be run overdriven much of the time. Fully bypassing the cathode on the first gain stage and using a smaller input grid resistor reduced hum and hiss quite noticeably. I did need to reduce bottom end with a smaller coupling cap before the clipping stages to keep the tone defined similar to the original design. But in my mind there's no going back. I like the reduced noise A LOT and will probably always use a fully bypassed first gain stage in future designs for that reason. So my vote is for reducing the coupling cap size. OTOH... There are a couple of respected designs that DO use a partially bypassed first gain stage. The amount of extra noise certainly isn't on the order of what you might get with a dirt box at the input. As guitar players we're pretty much use to added noise with gain. So what is acceptable for the tone you prefer is (has to be) up to you. Try it both ways and decide

I do agree that a considered reduction of LF through the circuit won't detriment the clean tone but greatly improves the clipped tone in a 5e3 circuit though.

[Chodrick1]

What do you mean by partial cathode bypass?
Right now I have a 1.5K resistor in parallel with an 8uF cap. I like the sound enough, however I believe it could be better.

[Chuck H]

An 8uf only cuts about .3dB (note the decimal ) at 80Hz. A 25uf cuts .2dB so there really shouldn't be much tonal difference between those two values (but see * below). With the exception that the 25uf should eliminate a little more 60Hz hum if it exists there in any objectionable amount. If you're going to partially bypass (just using a smaller cap value than would bypass all audible frequencies) you really need to get to 4.7uf or smaller to get a real difference. Pay attention to any increase or decrease in hum levels. If you find that hum becomes objectionable with the smaller values you can always experiment with reducing the decoupling cap value instead.

*This is according to everything you will read on the matter. In my own experience you CAN hear a difference between 25uf and 8uf. Sometimes I think the hard line of 80Hz for the lowest frequency on the guitar or the actual knee frequency of these bypass cap values is taken too stringently. Even when playing clean, if you combine two notes you can get a sum/difference tone at a frequency lower than 80Hz. There may also be small distortions and overtones analogous to the fundamental that are at a lower frequency. It's also speculated that the poor performance of guitar speakers negates frequencies below 80Hz. Yes it does, but not completely. My point is that "I" do know you can hear a difference because I have too. So take any advice to the contrary with a grain of salt.

[Chodrick1]

Alright, I can get a hold of something like that. I auditioned the 8uF cap by placing some leads with aligator clips from the cathode to ground and swapped the two values. The 8uF sounded a tiny bit cleaner, so I went with it. I good majority of the time I will look at other fender circuits and compare designs to get some idea of what certain components may change my sound. But more times than not I have found that the sum of all of the changes are what change the amp the most.

I will for sure add that to my parts list, I have seen a lot of people using 4.7uF bypass caps. What effect would completely removing the cap from the cathode and running that signal through the 820Ohm or 1.5KOhm resistor. I imagine it would remove almost all of the gain from the preamp. This is how the Champ and if I remember correctly, the Tweed Princeton both were built.

[Chuck H]

There would be a reduction in gain. By halving the cathode resistor you do raise the gain back up a bit, but not as much as a bypass cap does. Changing the cathode resistor also changes the bias (operating point) of the tube. There would be a tonal difference relative to symmetry of any clipped wave form and the hotter bias would also cause that stage to clip slightly sooner. But that only counts if that stage clips at all.?. And, in case it hasn't already been implied, a bypass cap across an 820R resistor has less affect on the gain of a stage than a bypass cap across a 1.5k resistor. There is also a change in the affected frequencies for a given cap value in either circuit. If this is starting to sound daunting, it's really not. Just keep reporting/corresponding and experimenting. Some things will clarify. And we DO want you to be happy with your amp