Question about Plexi 4 Inputs

[psychepool]

I plan to rebuild my existing 1-channel Deluxe Reverb to 5F6A/JTM45/SLP circuit.

Since I built it with a tight size, it is difficult to fit four inputs, so I am considering building it with a single input instead.

It is not that there is absolutely no room for four inputs, but the jacks would have to be placed very close together, so I think it will be very inconvenient when plugging them in.

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My idea is as follows:

I plan to implement the high/low inputs for each channel using DPDT on-on switches like this,

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and then distribute the signal to each channel using a switch from a single input jack like this.

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I intend to keep both channels constantly connected, and when I want to use only a single channel, I plan to set the volume to 0.

Will there be any issues with this? Alternatively, please let me know if there is another good way to properly imple

[Dr Tony Balls]

That will work fine but it seems overly complicated to have two high/low switches. Just have one switch and the output connects to both grids. I do this all the time but with just one "high input" jack.

[sluckey]

I think you are over-thinking this. Here's what Mark Huss and I do...

https://sluckeyamps.com/6v6plexi/6v6plexi.pdf

No need for hi/lo switching. The volume control on the guitar will accomplish the same thing.

[Colossal]

What Steve and Tony said. Simpler is better and less noise without the switching and extra wire. Having the two channels always on is great to shape the base tone and then you apply the polish with the tone stack.

[nuke]

Yeah, keep it simple. It will work fine.

[psychepool]

That will work fine but it seems overly complicated to have two high/low switches. Just have one switch and the output connects to both grids. I do this all the time but with just one "high input" jack.

Does this mean switching both channels to High/Low simultaneously? A standard channel jump also appears to be similar to using both with the inputs set to High.

I think you are over-thinking this. Here's what Mark Huss and I do...

https://sluckeyamps.com/6v6plexi/6v6plexi.pdf

No need for hi/lo switching. The volume control on the guitar will accomplish the same thing.

Yes, since I have almost no experience using 4 inputs, I do have a goal of experiencing that through this build. About 15 years ago, when I didn't know much about tube amp circuits, I did build one using that circuit by Mark Huss. Since I did it when I didn't know much, I don't really remember how it went.

What Steve and Tony said. Simpler is better and less noise without the switching and extra wire. Having the two channels always on is great to shape the base tone and then you apply the polish with the tone stack.

Whether using just one channel or both, is the Low input generally not used?

Yeah, keep it simple. It will work fine.

Yes. I see. Is 4 inputs not very useful to begin with? If I just follow Mark Huss circuit, will there practically be no difference in usage compared to the 4-input version?

[sluckey]

Whether using just one channel or both, is the Low input generally not used?

I've never used the low input. It's probably useful if you have a guitar with hot pickups or on-board preamp.

Is 4 inputs not very useful to begin with? If I just follow Mark Huss circuit, will there practically be no difference in usage compared to the 4-input version?

Today I only need one input. But when the original Plexi first hit the market, it was not uncommon to see more than one player plugged into an amp. If I had an original 1987 Plexi I would always plug my guitar into channel 1 high input and have an external jumper connected between channel 1 low input and channel 2 high input in order to use both preamps. But for a homebrew amp I prefer to use one input jack and do the jumper internal to the amp. If you don't want to use both channels just turn the volume pot to zero on the channel you don't want.

I bet that once you jump the channels you will like the sound much better and will never go back to single channel.

[Colossal]

Whether using just one channel or both, is the Low input generally not used?

Correct. Usually the high treble input only, or, both high inputs jumpered.

[Roe]

to minimize resistive hiss, lower the 68k grid resistors. and skip the low inputs. With the famous split-cathode lead circuit introduced in 1968, the normal channel becomes very dark, muddy and unusable. The bright channel by contrast becomes so bright that many use the normal channel instead or mix the two. It is better to adjust the bright channel to your purposes, e.g. by making bright caps switchable

[Colossal]

to minimize resistive hiss, lower the 68k grid resistors. and skip the low inputs.

Definitely. 10k-33k.

With the famous split-cathode lead circuit introduced in 1968, the normal channel becomes very dark, muddy and unusable.

Completely agree. The 220uF or 330uF stock bypass cap and 0.022uF coupling cap sounds like a tuba; totally unusable. I either lower the bypass cap to 3.3uF and add a small bright cap, or just jumper the channels and dial in a little Normal channel to fatten the sound up.

[xtian]

+1, permanently jumpered channels, mix with the volume pots. Easy.

[psychepool]

It seems 100% of people are skeptical about 4-input. I am skeptical as well. However, since I have never properly used 4-input before, I felt that if I didn't try it in this project, I would remain curious forever, so I wanted to approach it in a similar way. But after hearing your opinions, it seems I don't need to try it. Thank you for your reply.

Whether using just one channel or both, is the Low input generally not used?

I've never used the low input. It's probably useful if you have a guitar with hot pickups or on-board preamp.

Is 4 inputs not very useful to begin with? If I just follow Mark Huss circuit, will there practically be no difference in usage compared to the 4-input version?

Today I only need one input. But when the original Plexi first hit the market, it was not uncommon to see more than one player plugged into an amp. If I had an original 1987 Plexi I would always plug my guitar into channel 1 high input and have an external jumper connected between channel 1 low input and channel 2 high input in order to use both preamps. But for a homebrew amp I prefer to use one input jack and do the jumper internal to the amp. If you don't want to use both channels just turn the volume pot to zero on the channel you don't want.

I bet that once you jump the channels you will like the sound much better and will never go back to single channel.

As I always think, circuits created around this time were released before the usage and playing techniques we know today were established, so the features they provided do not seem to be always useful. In this build, I plan to implement the 5F6 and JTM45 in addition to SLP, but I expect that both channels will be used simultaneously at all times in those modes as well, just like in SLP mode.

Whether using just one channel or both, is the Low input generally not used?

Correct. Usually the high treble input only, or, both high inputs jumpered.

Yes. If I want a low distortion setting, simply lower the volume on the amp or guitar. I will proceed with a single input, like the Mark Huss circuit, without any separate switching. Thank you.

to minimize resistive hiss, lower the 68k grid resistors. and skip the low inputs. With the famous split-cathode lead circuit introduced in 1968, the normal channel becomes very dark, muddy and unusable. The bright channel by contrast becomes so bright that many use the normal channel instead or mix the two. It is better to adjust the bright channel to your purposes, e.g. by making bright caps switchable

I have often seen 33K resistors used for this type of input, but based on my experience using both, I didn't find any significant difference in sound. Is there any particular difference between using 68K and 33K, or resistors with lower values?

Although it is a memory from a build I made 15 years ago, I did get the impression that the normal channel of the SLP is difficult to use unless it is mixed with the bright channel. Actually, I plan to use this amp as a bass amp as well, and it seems the normal channel would be useful for bass.(Of course, using the bass in Bassman mode would probably feel the best.)

If I have time, I will draw out the circuit for my build plan.

to minimize resistive hiss, lower the 68k grid resistors. and skip the low inputs.

Definitely. 10k-33k.

With the famous split-cathode lead circuit introduced in 1968, the normal channel becomes very dark, muddy and unusable.

Completely agree. The 220uF or 330uF stock bypass cap and 0.022uF coupling cap sounds like a tuba; totally unusable. I either lower the bypass cap to 3.3uF and add a small bright cap, or just jumper the channels and dial in a little Normal channel to fatten the sound up.

What are the benefits and sonic differences obtained from lower grid resistance values? I have tried using 33K capacitors instead of 68K capacitors in a circuit that uses 1M/68K capacitors (connecting 68K capacitors in parallel via a toggle switch), but I did not notice any change in sound. In that case, does lowering the resistance value only offer advantages?

Since I plan to switch between the SLP and the 5F6A / JTM45, I do not intend to change the bypass and coupling capacitors. My goal is to make the circuit as close to its original state as possible. I am thinking of adapting to it as much as possible, even if there are unsatisfactory aspects. I will share my plans soon.

+1, permanently jumpered channels, mix with the volume pots. Easy.

I decided to do so after listening to many people's opinions. Thank you.

[Roe]

to minimize resistive hiss, lower the 68k grid resistors. and skip the low inputs. With the famous split-cathode lead circuit introduced in 1968, the normal channel becomes very dark, muddy and unusable. The bright channel by contrast becomes so bright that many use the normal channel instead or mix the two. It is better to adjust the bright channel to your purposes, e.g. by making bright caps switchable

I have often seen 33K resistors used for this type of input, but based on my experience using both, I didn't find any significant difference in sound. Is there any particular difference between using 68K and 33K, or resistors with lower values?

Although it is a memory from a build I made 15 years ago, I did get the impression that the normal channel of the SLP is difficult to use unless it is mixed with the bright channel. Actually, I plan to use this amp as a bass amp as well, and it seems the normal channel would be useful for bass.(Of course, using the bass in Bassman mode would probably feel the best.)

If I have time, I will draw out the circuit for my build plan.

to minimize resistive hiss, lower the 68k grid resistors. and skip the low inputs.

Definitely. 10k-33k.

With the famous split-cathode lead circuit introduced in 1968, the normal channel becomes very dark, muddy and unusable.

Completely agree. The 220uF or 330uF stock bypass cap and 0.022uF coupling cap sounds like a tuba; totally unusable. I either lower the bypass cap to 3.3uF and add a small bright cap, or just jumper the channels and dial in a little Normal channel to fatten the sound up.

What are the benefits and sonic differences obtained from lower grid resistance values? I have tried using 33K capacitors instead of 68K capacitors in a circuit that uses 1M/68K capacitors (connecting 68K capacitors in parallel via a toggle switch), but I did not notice any change in sound. In that case, does lowering the resistance value only offer advantages?

Since I plan to switch between the SLP and the 5F6A / JTM45, I do not intend to change the bypass and coupling capacitors. My goal is to make the circuit as close to its original state as possible. I am thinking of adapting to it as much as possible, even if there are unsatisfactory aspects. I will share my plans soon.

you get less noise (resistive hiss) and thereby an improved signal/noise ratio.

With the jtm circuit, the normal channel is usable. A forgotten trick is to adjust the volume of the unused channel. this affects the mixing of the channel and how the bright cap over the 270k mixing resistor affect both channels by affecting the voltage divider

[psychepool]

you get less noise (resistive hiss) and thereby an improved signal/noise ratio.

With the jtm circuit, the normal channel is usable. A forgotten trick is to adjust the volume of the unused channel. this affects the mixing of the channel and how the bright cap over the 270k mixing resistor affect both channels by affecting the voltage divider

So, the mix resistance of unused channels effectively acts as grid leak. And setting the volume differently would mean changing the grid leak. Is that correct?

[Roe]

you get less noise (resistive hiss) and thereby an improved signal/noise ratio.

With the jtm circuit, the normal channel is usable. A forgotten trick is to adjust the volume of the unused channel. this affects the mixing of the channel and how the bright cap over the 270k mixing resistor affect both channels by affecting the voltage divider

So, the mix resistance of unused channels effectively acts as grid leak. And setting the volume differently would mean changing the grid leak. Is that correct?

Grid leak is one issue yes but I was mentioning another issue, namely how the channel mixers work as voltage dividers together with the channel volume(s). Resistance to ground is affected by the unused channel. Max gain is found with the unused channel around 7-8 and the used channel at 10.